Off the top of my head: Once you get past $50/million a year, most of those people are going to be getting money from returns on capital rather than from salary. So how do you cap that? If someone invests $1 million in a start-up and eventually that stock is worth $500 million, do you stop them from selling it? Do they only get to sell $50mil a year?
Even just looking at salaries, do you limit what residents of the US can make, or what US companies can pay people? Then you've got a ton of possible workarounds like living abroad or taking deferred payments, etc. Of course the "it's complicated and you'll need a lot of extra rules to make sure it's not abused" is a criticism of almost any policy, and by itself it isn't a good argument against it. But in this case, it's just not going to happen and higher taxes would probably address the core problems anyways.
It's not that people get rich so much as capital keeps accumulating at the top to the detriment of the rest of society.
