All of that money came from somewhere. Do you wonder why most of your local retailers went out of business, and you can't visit a local bookstore or camera shop anymore? It's related to why Jeff Bezos has so much money that he has nothing better to do than shoot stuff into space. Preferably, he will shoot bigger stuff farther than Elon Musk, Richard Branson, or Paul Allen.
It could also be used somewhere else. Is shooting suicidal volunteers at Mars really the best use of billions of dollars?
Solving difficult engineering problems is usually good for the world.
Adam Smith directly says vanity is what took down feudalism. We surely dont want our billionaires to not even spend the money they have!
Printing money and setting it on fire would employ printers and firefighters. I'm not sure Billionaire Space Wars are a net harm to society, but they're pretty silly.