The super-rich don't get much of their income from wages, where there is withholding of a bit more than you're likely to owe if the wages are your only income. They get it from interest, dividends, and running a business. There is usually not withholding for those, so they're supposed to make quarterly payments. But the Republicans cut IRS funding years ago, so it didn't have the resources to go after them if they just stopped paying. So a lot did just that.