Because being somewhat of a workaholic or craving power / money / achievement is how they got to be wealthy in the first place. This is roughly akin to asking "why don't strong and fit people stop going to the gym?"
I enjoy the act of exercising and the immediate effects following excercise. Most people that are very into fitness or exercise enjoy it as well.
Its the same thing for people who are serious about the gym. You can deadlift 600 lbs? Cool, now its time to try to get to 650. If you hit 650 it is time to plan for 700.
You know, in the beforetimes, I went to the gym and asked myself that question a lot. (Just usually more in the context of grumbling "why didn't this incredibly buff dude unload all these plates from this machine")