Because it leads to bad economic choices by the rich and that doesn't actually help anyone. Just recently we had a discussion on how those in Hollywood avoided the high marginal rates:https://www.bloomberg.com/opinion/articles/2019-01-29/hollyw...

With high marginal rates, the tax shelters the Hollywood rich invested in were less productive investments. Yes high marginal rates will lead to more CPA hours to get around the high taxes, but that is just an example of the Broken Window fallacy. When there is no way around them, very high rates will lead to dead weight losses to the economy as economic activity won't be done.