There are plenty of legal ways to evade (or at least not pay) taxes as well, though. An easy way, for example, is to store wealth in the form of stock; generally you don't pay taxes until you sell, so increased value is ignored until then, but once you have that many assets you can borrow money using the stock as collateral, and in some cases I believe you can even write off the interest. All of this is perfectly legal, and I'm not sure if it can or should be "fixed", but a lot of cash that might otherwise be taxed simply isn't.

For that matter, they could also buy a piece of art for a million dollars, a year later get it appraised as worth $40 million, donate it, and then write off $39 million. Again, as far as I know this isn't strictly illegal, but it can be used to evade taxes.