Chess is a taxing game in many aspects. It doesn't help when your losing your mind because of chess, or also that you are going to be "tainted" with some future lose in chess because your losing your mind
why did bobby Fischer quit chess?

I never knew there were so many Bobby Fischer experts on these forums with such knowledge of little known facts.

he was too afraid to lose his title.. he ran and hid. i saw a documentary how he became the world champion. he was in a seclusion in a house very far from people. he was like a government project.. because of the cold war between usa and russia..
he was already kind of OCD person and those too much pressure drove him crazy i guess.
later on he hated us government and changed citizenship after many incidents..

TL;DR He got bored of chess.
he just hated how chess was evolving, takes too much work, theories and memorization was all it takes to be better or at the highest level, no creativity. there's an clip where he talked about this on yt. maybe it just wasn't fun winning by what you learnt in theory v.s. looking at the board and inventing moves. it became work, not play. i remember someone saying bobby fischer was just a genius who happened to play chess, and it might be true in the way that top chess players know about the bore of memorizing new theory cause it's their job but in terms of vocation it is not the most fun and evolutionary kind of thing. theory beat his genius. what he doesn't realize is that theory beats everything and is in every job or vocation to begin with.

But as far as dropping off the chess world for 20 years nobody predicted that, and Fischer left no clue as to why.
Before the 1972 match vs Spassky Dr. Reuben Fine, world-class chess player and licensed psychoanalyst who knew Bobby well, predicted that should he win the world championship he would be so fearful of losing the proof of his superiority that he would never play chess again.

Having scaled the highest pinnacle of the chess world, Fischer had nowhere left to go. Like Alexander, he had no more worlds to conquer. He had been deserted by his enemies.
From that point on, there could be no more major victories... just a rear-guard defense of his World Champion title against successive waves of younger and more motivated challengers.
From the point of view of his own ego, Fischer had nothing to gain and everything to lose by remaining on the chess throne until he was forcibly deposed by a successful challenger.
Would Karpov have beaten Fischer in 1975? I think not. Not without some sort of mental or emotional collapse on Fischer's part. How about in the following cycle, in 1978? Much more likely, yes. By that time, Karpov would have had three years more tournament experience, and Fischer would be three years older.

At the end professionally he had nothing to prove.but even in the wilderness he may of acquainted a game or two the way he started in the chess club. Afterwards where he settled it seems caught up in the ramblings of his persecution. Possible similar to Morphy psychology speaking some say but look what he gave us and gave the game a shake up with new ideas.

Fischer was possibly the most influential person that came to the game his ideas were groundbreaking that made suspicion at first but he was always a brain and pioneer that even had a physical fitness routine. I'm not sure if he went the way of some changers of the game but he was certainly under a lot of pressure for just playing. His match with Spassky coming from behind was thought impossible after the blunder and forfeit no wonder Spassky respected him especially at that time of history.

There has been a lot of discussion with chess genius players that fall from lofty towers but I think he was not able to put away his demons and opinions expressed that damaged him. But the mind behind all that! Possible still as said most influential person who played.

Fatigue possibly.
As much as I love chess it's tiring. I've been known to sleep for 15 whole hours after spending too much time binge-playing rapid.
The real question is has Magnus Carlsen retired from serious chess? I've been searching when he will play in the next classical tournament and could find no results. By default only playing blitz would mean he gave up his title as world champion and serious chess other than simple 5 minute skittles at about the same age Bobby Fischer did. Could the fact that he didn't win a single game in his last classical tournament indicate he realizes he no longer has the stamina to compete at the top level while living a playboy lifestyle?
Chess amplified them and made them worse.
There's no way of knowing that.
You could just as easily argue that chess gave him something to focus on and helped him keep his life together for the first 30 years... because after he quit he only got worse.
I guess so.