Many people don't even have the privilege yet to vote up (15 rep) or vote down (125 rep (100 on meta.stackexchange.com)). So I'd guess that for users that eventually newly gain the voting privilege, it might just a "habit of omission" that stuck. It could also be something worse (if users who haven't earned the voting privilege yet feel like the voting tooltip is telling them their votes don't matter, it could become a habit to not vote). It could be that voting doesn't have as big as an effect on UX as users might like (see Add an option to sort answers by the viewer's previously cast votes).
If you only vote on answers and not questions, the system does give a popup/toast reminder: "you haven't voted in a while; questions need votes too!"
The best way to remember to vote is to practice! You can practice by helping curate the site by voting on things. One way is to help with the review queues (you'll need to gain that privilege first though). The review queues encourage you to vote on posts. You don't need the review queues to curate the site. You can just sit on the newest, interesting, or active questions pages.
If you want to vote in a principled way, learn about what qualifies good and bad posts by reading the Help Center pages on questions: /help/how-to-ask and (where applicable) /help/minimal-reproducible-example. Make sure also to flag or close-vote where appropriate (see also /help/on-topic and /help/dont-ask).
The vote tooltip says: "This question shows research effort; it is useful and clear" and "This question does not show any research effort; it is unclear or not useful". Do note that research effort is primarily about doing research within the Stack Exchange network (although various sites may have varying standards that you can learn by lurking and reading their meta posts), and that for technical, reasonably-scoped how-to questions, implementation effort is not a requirement.
As Glorfindel mentioned, you can motivate yourself by trying to work toward earning voting badges (of which there are many!): supporter, critic, suffrage, vox populi, civic duty, electorate, and sportsmanship