Because we grew up internalizing external expectations. You have to unlearn that if you don't want to feel pressured into completing the last three achievements, but that's hard AF especially for people who are masking a lot. And until then you feel guilty for not completing a game because you missed an achievement or didn't find all collectibles.
Completely with you. But then it is not helpful, to also pressure each other online to complete stupid external stuff, when someone is clearly managing to just really enjoy the actual game.
There's only one achievement of the ones left that I actually care about and I haven't gotten around to attempting it yet. I've mostly been playing modded levels and doing any% speedruns.