Because at least for big moonshot ideas, it's much easier for one billionaire to bankroll some crazy idea everyone else thinks is a waste of money.
And even if you could gather the funding, you now have to keep a few dozen investors happy versus just pursuing your idea however you want.
And sure the money doesn't disappear, but would you really expect an Amazon warehouse worker who now makes $70k instead of $35k to be investing in highly risky ventures? I mean, they wouldn't even qualify to invest.
As mentioned, there are other vehicles to obtain financing. Secondly, when billionaires disappear, their wealth doesn't disappear with them. Not sure what you are asking.