We know Michael Phelps as the single most decorated Olympic athlete of all time. Winning medal after medal, he has shown a level of focus and determination that only a nearly perfect specimen can have. And when an athlete has such a passion for his/her sport -- and he/she is amazing at it -- it's very hard to give up. Most can't do it.
Look at athletes like Mario Lemieux and Michael Jordan. They were pros that left at the top of their game and wound up returning for an encore. Their sport becomes an addiction. A swimming nut like Phelps won't be out of the water for long.