You statement makes it sound like they aren't currently doing those things already. Can you point to some data that shows they aren't donating to charities and aren't diversified? I can understand if someone like Bezos is highly invested in the company he founded but overtime he will be like Gates and hold a small portion of his portfolio in Amazon and the rest broke up across many companies and charities.

Then I guess a wealth tax is not needed, billionaires are already giving away all of their money or starting new companies through direct investments, so they're no longer billionaires.

If you have a billion dollars and you invest all of it, you're still a billionaire (unless your investments tank). No billionaire keeps all of his money in cash.

If a wealth tax isn't going to change anything, then what's the point? Do we really believe that billionaires are keeping their money in scrooge-mc-duck-esque vaults rather than investing it in diversified portfolios, and that only a wealth tax (rather than the promise of returns on investment) will force them to invest?