No, they're doing this for the power that the number can give them. After we create a system that rewards the people who maximize their wealth at any cost, we shouldn't be surprised that eventually we get a group of people concentrated at the top whose lives revolve around doing that one thing. We also shouldn't expect there to be some arbitrary amount of wealth and power that trips the failsafe in their brains and reverts them to the state of mind of an average human being. They will keep going, the goal is owning as much of the world as is feasible. Power is an intrinsic motivator. Most billionaires only sit on their hands and watch their number go up, so for those it's immediately apparent. For the exceptions, like Musk, it's a bit more complicated, but the desire to advance human spaceflight likely comes in far behind the desire to out-do everyone else, get your name in history and own a brand new unexplored frontier under your own name.