People and companies would systematically move overseas. People would renounce US citizenship.

There are many, many things that make that not as easy or desirable as you make it sound. As an extreme example, the current tariffs. Renouncing citizenship means people no longer have a right to work or travel in the US without a visa. That makes continuing business harder, and keeping family connections harder.

Keep in mind a 91% top marginal tax rate isn't an effective rate of 91%, it would only affect money over some theoretically large amount. If you made $10 million gross, it doesn't mean you take home $900k. Depending on how the rate tiers are set up, you're probably taking home multiple millions of dollars, and you can get a lot more bang for your buck with some good donating (since the higher the rate, the less it affects your bottom line). Also, maybe there's investment opportunities that allow for it to be put back into businesses as VC that also delay the tax, and allow for it to be put to use.

So the question is, would you uproot your life and move away from friends and family and make it harder to do business so that instead of taking home $4 million of your $10 million gross you take home $6 million or $7 million? Given how little utility that extra money likely has, I'm not sure many people would (especially since the bragging rights for how much they made the prior year remain unchanged).