Why do people hide the assumption contained in the philosophical zombies question/idea?

You say that the first para of the SEP article include the words "[Zombies] are exactly like us in all physical respects but without conscious experiences". Clearly the 'but' implies that we (ie humans) have conscious experiences. If you consider that to be a hidden assumption, so be it. Others might consider it a matter of fact.

Claims of hidden assumptions can be made in relation to almost anything. Suppose I asked you to imagine a planet that was locally identical in every respect to Earth, except that it was flat. Would you say there was a hidden assumption that the Earth was not flat? If I asked you to imagine a white Volkswagen Beetle that was identical to any other white Volkswagen Beetle except that it had a mind of its own, would you say there was a hidden assumption that white Volkswagen Beetles don't have minds of their own? I am as utterly convinced about my having conscious experience as I am about the Earth not being flat and Herbie being a fiction, so to me there is no hidden assumption in the SEP article. If you are convinced that you are a zombie, then to you the SEP article might seem utterly bigoted nonsense that begs a very important question.

So my question is, does the philosophical zombies question/idea contain a hidden assumption and why (if/because it does) is this assumption (that normal humans are conscious and have qualia) not explicitly stated when the zombie question is asked?

Yes. All questions do. And all answers. And all arguments. This was noted and explicated on by Quine with his notion of confirmation (epistemological) holism in his exploration of theory ladenness which has a perceptual and semantic aspect. While the discussion is had among philosophers of science, the notion that there is a holistic aspect to epistemological thinking permeates contemporary philosophy of language with its focus on expressiveness, productiveness, and compositionality. Semantic epistemological holism is the thesis in its moderate form that propositions are only meaningful within the context of the situation. Thus, there is the larger body of text (endophora), by examining references outside of language (deixis), refering to meaning not specifically encoded (implicature), and paying attention to the intentions of language (Sprachspiel). This is what Quine may have been signalling when discussing the notions of web of belief. Every argument made has hidden assumptions, because no one spells out one's worldview before crafting language. The question becomes is it necessary to include the assumption? The one you point to doesn't seem necessary since it is almost universally presumed to be true.

So, is it unstated that human beings have consciousness and experience qualia. Yes. Is this surprising? No. The objective of philosophical zombies argument is to challenge that intuition and force the thinker to deal with the notion that other human beings might not have minds. This isn't isn't revolutionary. It's solipsism in a modern language.

In Ancient Greece, three important epistemological problems unfolded themselves. The first is the question of the existence of knowledge. Why shouldn't we be radical skeptics? The second is the question of the difference between appearance and actuality. Should we accept naive realism? And the third is how do we know other minds exist if we only can introspect our own? This is the problem of solipsism. Needless to say, most modern thinkers move beyond Pyrrhonism, naive realism, and solipsism and accept knowledge exists, accept representational theories of mind, and accept that we may infer other minds exist. Thus, the problem of the philosophical zombie is an attempt to push back on the last two to accentuate that there isn't consensus on mind-body duality, which is another problem that seems to arise.

Why do people hide the assumption contained in the philosophical zombies question/idea?

So, the assumption is not hidden. It's just so obvious it isn't mentioned in these sorts of conversation, because the notion that humans have consciousness is powerfully intuitive. According to a naturalistic approach to epistemology, the reason for that is straight forward: human beings are genetically enabled to generate theories of mind and to use collective intentionality. Like breathing and using our opposable thumb, it's simply natural to understand and collaborate with other people; in fact, it is thought by some researchers that autism is a condition that is caused in part by neurodiversity leading to mind-blindness. I suspect most psychiatrists would consider radical solipsism, an unfettered inability to believe in other minds, a presentation of a symptom of a mental disorder. People who sincerely believe they are Jesus, or Elvis, or aliens, or the only person really alive behave in ways that confirm it, and lead to social situations that support therapy and institutionalization in the extreme.

The sort of people who engage in vibrant debate about mind-body duality, reducing the mental to the physical, and can discuss intelligently eliminative materialism and subjective idealism presumably all have the intuition that other people have conscious experience. Thus it's not usually explicitly spelled out for parsimony.

If actual humans are philosophical zombies, then none of them have the concept of qualia (if they have any concepts at all). There would be no point in asking a question about possible qualia-free lookalikes of qualia-ful beings, then.

So, either (A) the ability to ask the question about qualia and zombies is itself disproof that we are zombies (or else we couldn't ask the question), or (B) the question is meaningless. To actively make the opposite "assumption" in the OP case would nullify the pragmatic ground for conducting the thought experiment at all.

So, not so much that the assumption is hidden, but that making the assumption is required to ask the given question in the first place. Dennett wouldn't be factually correct about his own theory in saying, "My theory is that actual people, including myself, are philosophical zombies." If he doesn't believe in qualia (or if his concept of qualia is empty), then he's just not talking about things that other people talk about when they proclaim qualia; he's using the same words but his presuppositions fundamentally redirect those words' meanings.

"Why aren't we zombies?" There's the assumption that we aren't, but it's not acknowledged.

You have to understand, the language-game 'Philosophical Zombies' is above all a thought experiment. Whether we personally are not zombies, or whether zombies are possible, are not subject to assumption or observation - but, definition in this context. The thought experiment begins there, with looking to consequences if the definition is allowed to stand. So really, Dennett conducts a sleight-of-hand, which personally I don't think stands scrutiny, he just says what we call inner experience isn't really that, when clearly definitionally it is - a la cogito.

P-Zombies is the parallel of the Turing Test, where there is the recognition 'all we have' for other humans seems to be that we pass such a test of acting conscious, and so allows us to focus on the gap between our experience of ourselves and how we experience others, and examine our intuitions about ourselves and others.

To me the critical piece for this topic, which I find all too frequently bizarrely and to my mind inexplicably neglected, is The Private Language Argument. Take Wittgenstein's points about pain, a topic you raise:

"What would it be like if human beings showed no outward signs of pain (did not groan, grimace, etc.)? Then it would be impossible to teach a child the use of the word 'tooth ache'." Well, let's assume the child is a genius and itself invents a name for the sensation! But then, of course, he couldn't make himself understood when he used the word. So does he understand the name, without being able to explain its meaning to anyone? But what does it mean to say that he has 'named his pain'? How has he done this naming of pain?! -PI 257

He has a lot more to say on this, and the full Private Language Argument was not strictly developed by Wittgenstein but from his work.

This quote though points us at intersubjectivity. We do generally, if we are not solipsists, begin with the assumption other humans are like ourselves, and, that is functional and useful, in constructing our world and reality - like so: Indra's Net. That is, whether or not we consider other humans to 'really' be like ourselves, if we at least 'act' like they are, then we can engage in this peer-to-peer reality where our experiences are named and organised through engaging in language games, and we greatly extend our experiences and understanding by acting like other people are like ourselves - and, beliefs to the contrary aren't really functional. Through language tools like 'chunking' and assembling 'salience landscapes', and forming 'explanatory layers' that use those, we focus on the bits of the world where we can have influence, enabling our mental model of reality to give us 'cognitive grip' on our experiences. See detailed discussion here: According to the major theories of concepts, where do meanings come from?

The Turing Test in practice is subject to endless iterations and reforming, because despite superficial plausability in practice we just don't feel that engaging in text-based chat does indicate the presence of inner experience, though it can apparently fool some people some of the time (I'd suggest this was actually done in bad faith for attention, which it got, given the substantial editing of the chats involved). But, the idea of the Turing Test points towards the shift from thinking about what subjectivity 'is' to what it does for us. What we really look for in a Turing Test is evidence of subjective experience like our own, manifested by mirroring, empathy, and sponteneous responses to internal experiences like our own. When we think more deeply, these may not be good criteria at all, and while we can point at structural/functional attributes like manifesting a strange-loop, there is a real risk of suffering occuring that we are blind to by only recognising human-like sentience, as discussed under Bostrom's 'mind crime'.

I would explain the deep roots of this gap, between the idea of the Turing Test and the practice, by looking to how we expect a deeper mirroring by other humans: for them to deduce things about our experience by queues like wincing, and that the assumptions involved in that pre-exist our capacities to make sense of our experiences by naming them, involved in language. We just can't begin to sift noise for signal, and begin to form tractable models of the world, without accepting it. Babies isolated from interaction die, babies deprived of verbal language develop their own (babbling, hand gestures, etc), and risk never becoming fully cognitively developed if the don't get human interaction (see children raised by wolves, or severely neglected). Human brains fully developing, and language, arise together. An AGI could potentially arise out of language itself, in the way AlphaZero generated a chess player, this is I think comparable to how some twins sponteneously generate their own language from the evolutionary trigger of babbling (associated with FoxP2 genes, plus honed by mirror neurons).

We can observe degrees of intersubjectivity with animals: How to define intelligence amongst animals

We can I think have experiences without language, new-born babies do, although they quickly learn to cry when hungry, reach when they want to be picked up, focus their eyes and make eye contact to show interest or ask for attention, mimicry, and so on - which in this context are language. With distinct developments in children's quality of intersubjectivity, like the ability to decieve others that usually develops around age three. Our observations themselves, rapidly become theory-laden, through the application of language. And a huge amount of human cognition and communication is about picking up implicit and contextual queues, which is highlighted by trying to communicate with computers, and we can relate evolutionarily to the neocortex (Dunbar Number) and the Default Mode Network. Simply participating in interactions with other agents, can begin to develop language.

I feel that the distinction in science between phenomena and theory, is a useful model for understanding the relation between personal experience (not private, but intersubjectively-informed) and language. Discussed here: Is scientific knowledge personal or general? We use tools like consilience and examining our cognitive biasees, to filter what about our experiences is intersubjective, and would be experienced by others in a similar way if they were in our situation.

We are enculturated into a peer-to-peer reality, but there is the capacity for a 'hard fork' or paradigm shift, if a new framing can take enough people into it's language game. This can help us understand the problems of interspecies communication.

The Stanford Encyclopedia of Philosophy article called "Zombies" (...) makes no mention of an assumption (...) that normal human beings (...) have qualia and conciousness.

The two sources that you yourself quote give the appropriate answer:

Unlike the ones in films or witchcraft, they are exactly like us in all physical respects but without conscious experiences -- Stanford Encyclopedia of Philosophy

A philosophical zombie argument is a philosophical thought experiment which conceptualizes a hypothetical being that is physically identical to and indistinguishable from a normal person, but does not have conscious experience -- Wikipedia

Zombies here are said to be like us, or indistinguishable from a normal person, except for their lack of conscious experience. This is enough for the reader to be able to infer that these authors assume that normal humans have conscious experience.

I don't see where is the problem. Philosophy is not formal logic. Readers are assumed to be able to infer what is left implicit when it is implied by what is said.

Zombies - SEP

The article in question (First published Mon Sep 8, 2003; substantive revision Sat Mar 25, 2023) explicitly states the usual assumption in the last paragraph of Section 1.

https://plato.stanford.edu/entries/zombies/

The usual assumption is that none of us is actually a zombie, and that zombies cannot exist in our world. The central question, however, is not whether zombies can exist in our world, but whether they, or a whole zombie world (which is sometimes a more appropriate idea to work with), are possible in some broader sense.

What is it like to be a bat? - WP

https://en.wikipedia.org/wiki/What_Is_It_Like_to_Be_a_Bat%3F

Thomas Nagel - "An organism has conscious mental states if and only if there is something that it is like to be that organism – something that it is like for the organism."

Philosophical Zombie - WP

https://en.wikipedia.org/wiki/Philosophical_zombie

Philosophical zombies are associated with David Chalmers, but it was philosopher Robert Kirk who first used the term "zombie" in this context in 1974. Before that, Keith Campbell made a similar argument in his 1970 book Body and Mind, using the term "Imitation Man."

Are There Philosophical Zombies?

Talk by David Chalmers

https://youtu.be/-UTlcF-OT8o

Phenomenal consciousness: The subjective experience of mind and world.

A being is conscious if there is something it is like to be that being.

A human is conscious. Dogs are probably conscious. Rocks are probably not conscious.

A philosophical zombie is a being that is physically identical to a conscious human being, but that is not conscious.

There is nothing it is like to be a philosophical zombie.

Conscious and Unconscious Human Cognition Is Explicit in the Definition of Philosophical Zombies

I am conscious of my inferences that other living humans are conscious, that living dogs are conscious, and that rocks are not conscious. I am conscious that it appears as if my inferences in this domain arise as the product of an unconscious process as described by Hermann von Helmholtz. I am conscious of my ability to doubt or question these inferences. If philosophical zombies exist, and if I have interacted with them, then they have fooled me into thinking they are human like me, and by definition I would not be able to detect the difference between a human and a philosophical zombie! The actions and behaviors of a philosophical zombie would be driven totally by some sort of unconscious cognition but by definition every human would infer that the zombie is conscious!

The short answer is yes. The assumption, the default position, is that humans have consciousness. Even as a thought experiment, I find the definition of a p-zombie unsatisfactory. Suppose someone slaps you and the p-zombie in the face for no reason apparently. You and the p-zombie cry out in pain and shout and swear at your assailant. The p-zombie feels pain just like you, but is not conscious of the insult implied in the slap. So why does it shout and curse just like you? The definition is inconsistent. To suggest that the only thing that I can be absolutely sure of, my consciousness, is an illusion is sophistry. Consider that the difference between consciousness and the illusion of consciousness is indiscernible. Then, employing Leibniz's principle of indiscernibles, they are logically identical.

You are right in supposing that there is something significant missing from almost all presentations of Chalmers' zombie argument, but it is not the issue you suppose; it is the question of whether conceivability does entail possibility.

I do not think one has to be a committed materialist to find something to be deeply skeptical of in the form of this argument. After all, conceiving of something seems to be merely the first step in finding out whether it is possible, and we are all well aware of conceptions that, while widely held to be true at one time, turned out to be impossible or incoherent. There is also the matter of the many unsolved conjectures in mathematics, such as the Collatz conjecture, where both their truth and falsity seem conceivable, yet these alternatives cannot both be true. Then there is the sense in which the premises that one rejects have to be conceived of in order to reject them. Furthermore, if there is any objectivity to the zombie question, then it would seem that we must accept their possibility merely on the basis of David Chalmers finding them conceivable, never mind what we think!

This concern cannot be dismissed by noting that, in ordinary usage, 'conceivable' and 'possible' have considerable semantic overlap, such that there are many cases where they can, in practice, be used interchangeably. While I suspect that this plays a big part in the easy acceptance of Chalmers' entailment premise, it amounts to question-begging via equivocation leading to a de-facto (if unacknowledged) conflation of 'conceivable' and 'possible'.

Chalmers is, of course, well aware of these issues, and has attempted to address them at length, such as in a 2002 paper appropriately entitled 'Does Conceivability Entail Possibility?' Here, he sets forth three different notions of conceivability, and combines them to produce eight different sorts. In the appendix, he reveals what sort works for the zombie argument: it is the premise that zombies are, in his terms, ideally primarily positively conceivable.

That 'ideally' is, just by itself, a highly significant qualification. Unlike his prima facie conceivability, where the premise in question is not immediately and obviously false, a claim of ideal conceivability is a claim that the premise remains conceivable after "ideal rational reflection".

Rather tellingly, Chalmers then considers several ways of defining what "ideal rational reflection" means, and finds difficulty with all of them. He writes "I will not try to give a substantive characterization of what good reasoning consists in, or of what counts as a cognitive limitation to be idealized away from. I suspect that any such attempt would end up being open-ended and incomplete." Well, yes, and this, of course, would carry over to the zombie argument.

Chalmers has developed this line of thought via two-dimensional semantics (see 'The Two-Dimensional Argument Against Materialism', 2009.) The argument has become even more complex but has not been able to either shed the need for the conceivability of zombies to be ideal or provide an uncontentious argument for it being so, and he resorts to offering arguments for its plausibility by invoking other, equally contentious, anti-materialist arguments.

For the purpose of answering your question, I think all this makes it clear that the zombie argument is very far from being the simple and straightforward knockout blow to physicalism that it might appear to be when presented in its two-premises-to-a-conclusion form.