How am I supposed to earn them if I don’t even know what they expect me to do for them? Why do these exist? I haven’t even encountered a game (not even a Fromsoft game, and those guys are notorious for being vague and not forthcoming with info even for guide books) where the hidden achievements were not already listed by a 3rd party somewhere outside of the game day 1, so why not just show them in the damn game?
Just as a quick FYI, on the PS4 (and presumably the PS5 too), if you click on any of the hidden trophies, you can press square (I think) to show the description of how to earn it. This wasn’t possible in the PS3 era, but it’s one of the upgrades they implemented in PS4. That’ll save you a Google search.
As for why, as others have mentioned, it’s mostly for spoilers or hiding easter eggs that are more fun if you find them naturally rather than going out of your way looking to earn a trophy.