It's no different than any other industry. No matter how passionate you are to start, it's the rare few that can maintain that passion over the years of losing seasons, heartbreaking playoff losses, injuries, etc. Even when you win it all, it's tough to build yourself up and try to do it again. It becomes a paycheque for most at some point.
That's what makes the Tim Duncan / Dirk Nowitzki types so exceptional. There's just no quit in them.