For example, take Jeff Bezos. His income isn't all that much, so you'd probably want to tax his wealth. Luckily, most of his wealth is from a public company, so we can skip the difficulty of valuing it (something a lot of other billionaires would have to deal with).
But now how do you collect the tax? Do you make him sell 95% of his shares of Amazon? That would certainly tank the stock, and then you would owe him a big refund for overtaxing him.
Or does the government just take his Amazon shares? So now the government would be the biggest shareholder of Amazon. Is that good? What does the government do with that stock? Does it start giving shares to government workers instead of paychecks? Does it pay off its debts with Amazon stock? Would the government's vendors even take that as payment?
I agree that wealth inequality is a problem, and we need to do something about it. I'm not exactly sure what or how though.