You're taxed on income, like anyone else, but the big difference is that wealthy people get liquid cash through tricks like borrow, spend, die. They also have other ways to gain wealth that don't involve a cash phase.

I'll get hate from some corners on this, but I'm coming to see the wisdom in not significantly increasing the tax rate on the rich, but instead broadening the tax base. For example, count tricks like borrow, spend, die as cash; make stock buybacks illegal; treat compensation in the form of assets as cash income at the valuation price when the transfer happens; count inheritances above $X million as income, cash income.