Most Billionaires are only Billionaires because of the value of the company they own a huge number of shares in, and they’re busy running that company which consumes all their time and energy. They also can’t cash out their shares (or at least not most of them) without risk of losing control of their company, so they don’t actually cash in on their vast wealth. Instead they’re happy to be a Billionaire on paper and “merely” live as a Millionaire.
Edit since I’m getting downvoted: yes the ultra rich can and do live off debt with their stock as collateral, but that gets you to the “merely” living a luxury lifestyle point. It’s not a viable mechanism for funding SpaceX, Blue Origin, Virgin Galactic, etc, which is the kind of “interesting stuff” this thread was talking about.