At some point money stops being about what you can buy and turns to a competition. You become addicted to the feeling of getting ahead of your rivals on the Forbes list, watching your companies stock price tick up after a good quarter, coming up with megalomaniac plans and then feeling happy when you can finally afford them even if you never actually wanted to pull them off.

Being a billionaire is a conscious decision. You can always choose to give away your shares to employees or sell them and donate the profit like Warren Buffett. If you decide you want to keep all of it for yourself that means you are a selfish person and probably a psychopath. And for these types making money is a goal by itself.