Why do you want that moon in Super Mario Odyssey?
Because I can't progress in the game if I don't? Because doing so unlocks no content and new gameplay? Because it's an intrinsic part of the
gameplay.
Now, if the achievements are built into the actual design of the game and provide functional rewards by unlocking them, that is a different story. Unlocking a new costume for completing the game with every character, or getting a new title / avatar for 50 online wins, or earning a powerful weapon for beating a secret boss. That sort of thing. That is adding something to the experience.
I rarely see this actually done, but it's enjoyable when it is.
The older Uncharted games are a good example of this. The out of game trophies mirrored in game milestones, and you earned things in the game for accomplishing them. Functional identical to the reward systems of the game itself. The recent Spider-Man also did this, with almost all the trophies in the game directly tied to progress and unlock paths. Very few gimmicks that you had to go out of your way to get, just for the sake of getting them.
Isn't the act of running and jumping as Mario fun enough? Why go out of your way for a hard to reach moon? You can't use it for anything in real life.
This is a false equivalence argument.
Because there wouldn't be any game if I didn't. It would be like asking me "Why not just randomly move the chess pieces around the board? Why plan out a strategy?" Because I will lose the game if I don't.
The inherent goal of every single video game ever made is to "beat" it. This is the basic concept, design and purpose of them by their very design, and why people play them. Without that element, it ceases being a video game at all.
Achievements give goals for a hobby based on achieving goals and permanent record of said goals.
That doesn't really answer my question though. I know what they do. I'm not attacking you. I'm simply trying to understand
why, on a psychological level, does it feel like a lesser experience to you without them?
What I especially do not understand is when people go out of their way to unlock achievements solely because they're there, and not out of any sense of enjoyment of the experience. So many times I hear about completionists getting annoyed or frustrated at games because certain achievements are tedious or extremely difficult or tied purely to a deliberate grind. In that case, all I can ask them is... why are you chasing after them then, if you're not even enjoying yourself?
That borders on obsessive compulsion to me. Putting physical and emotional stress on oneself for the sake of a virtual badge case.
The only game I've ever gone out of my way to get every achievement in is Bayonetta, and that's solely because I wanted to do all the content of that game anyway. Getting the achievement was only an aside to that experience. I wasn't seeking them out on purpose. Bayonetta 2 never felt like less of an experience to me for the lack of them.