I have a slightly different version of this. There are some other replies here to the effect of "people like to have a purpose in life". But surely "helping people share files" or whatever the Dropbox "mission" is can't be that inspiring, right? I mean, the point of starting a company like that is to make those billions. Maybe to do it successfully you need to delude yourself into thinking that mission is really important, I don't know.
But for me, if I had that kind of business success I'd cash out and go do something that feels actually important. Go get a PhD and take a crack at curing some obscure disease, or setting up a Dyson swarm around the sun, or making a great work of art, or thinking about how to improve elementary education a la Khan Academy; something like that. Then you get to have a real purpose and work toward something that most people don't just by virtue of the fact that it doesn't pay well enough, and you still get to spend plenty of time on your yacht!
The instinct to acquire resources does not have a limit. When homo sapiens evolved, such a limit would not have been a useful adaptation. There were no billionaires then.
Also it's nice to boss people around, probably for the same reason.
You and lots of people say would do something or other. I submit that it's different when you're actually in that position, as opposed to imagining it. Lots of us imagine we're going to go to the gym for years, and give up after a month or two.
Some people have made the (good in my opinion) point that this is actually survivorship bias and a lot of people actually do retire and are never heard from again once they have enough money. So people predicting what they would do aren’t necessarily that wrong, it seems that some people actually do that.
But for me, if I had that kind of business success I'd cash out and go do something that feels actually important. Go get a PhD and take a crack at curing some obscure disease, or setting up a Dyson swarm around the sun, or making a great work of art, or thinking about how to improve elementary education a la Khan Academy; something like that. Then you get to have a real purpose and work toward something that most people don't just by virtue of the fact that it doesn't pay well enough, and you still get to spend plenty of time on your yacht!