Since this feature is pretty trivial to implement, why isn't it more common?

Your premise is wrong. It’s not “pretty trivial to implement”. In fact, it brings a bag of problems.

Let’s have a look at the suggested “solutions” in the post:

• No precedence. The author himself says “Not using precedence rules is simply not an option.”
• Semantic aware parsing. Like the article says, this would require the compiler to have a lot of semantic knowledge. The article doesn’t actually offer a solution for this and let me tell you, this simply isn’t trivial. Compilers are designed as a trade-off between power and complexity. In particular, the author mentions a pre-parsing step to collect the relevant information, but pre-parsing is inefficient and compilers strive very hard to minimise parsing passes.
• No custom infix operators. Well, that’s not a solution.
• Hybrid solution. This solution carries many (but not all) of the disadvantages of semantic aware parsing. In particular, since the compiler has to treat unknown tokens as potentially representing custom operators, it often cannot produce meaningful error messages. It also may require the definition of said operator to proceed with parsing (to collect type information etc.), once again necessitating an additional parsing pass.

All in all, this is an expensive feature to implement, both in terms of parser complexity and in terms of performance, and it’s not clear that it would bring a lot of benefits. Sure, there are some benefits to the ability of defining new operators but even those are contentious (just look at the other answers arguing that having new operators isn’t a good thing).