People move on from previous endeavors. They let them go. And when you are competing at a level involving the highest standards in the world, whether it be Olympic sports, the NBA, NHL, NFL, pro boxing, MLB, there is room for practically nothing else, and you are likely to burn the hell out after some point. I mean burn out to the point at which you don't even want to participate anymore.
Has anyone seen what Bo Jackson and Mike Tyson look like now?
Many of these people probably want to experience things they could not while they were living and breathing and thinking about their endeavor around the clock. Relationships were put aside or not tended to, if not wrecked. Some did not have children. Friendships were strained. They didn't travel on a leisurely basis. And the list goes on and on. Maybe they now want to... well... you know... go home, watch a TV show, read some books, work on cars or do some other crafts.. and to hell with the gym?!