I mean, theoretically they were already taxed on it once. A wealth tax taxes accumulation, which happens after the tax on income.

In practice billionaires are taxed at about half the rate of minimum wage workers. I was so mad when I got my first minimum wage paychecks. I worked so hard for so little and got taxed at such a high percentage. It seemed deeply unfair that billionaires and corporations paid a lower tax rate than me.