Billionaires think differently about money than non-billionaires. Ordinary people use money to exercise control over their own lives. Billionaires use money to exercise control over other people's lives, and when that is your goal, a few extra billion dollars means a few extra tens of thousands of people you can put on payroll to do your bidding, whatever that might happen to be. It lets you do things like run for president or go into space. It's not my cup of tea, but it's not fair to call it "irrational". Some people just want different things.