Former tax professional here. The problem is that the billionaires aren't really billionaires. Elon Musk does not have a quarter trillion dollars in his bank account. His net worth is calculated from what other people think his holdings are worth. He cannot be taxed on this.

Unless someone is game enough to pass legislation enabling taxation of "unrealised gains" (while not allowing credits/offsets for unrealised losses), billionaires will continue not paying their fair share of tax.