There are a lot more poor people than there are rich people. It’s a game of numbers, and a slight increase on the middle class would often bring in more money than a substantial increase on the top percent.

Rich people also have a lot more loopholes they can abuse to pay less in taxes. Closing these loopholes could also potentially/occasionally disproportionately hurt lower or middle class people. They are possible to close, but (A) there’s always more to find, and (B) lobbying means there could be political incentives to not close them.

There’s also the arguments about raising taxes leading to innovation stagnating, or rich people moving to countries with lower tax rates. I’m not sure how much I buy those arguments.

That said, I’m not condoning these. In my country, I think we need to introduce more tax brackets. A doctor making less than $300,000/yr shouldn’t be in the same tax bracket as a CEO making an over $800,000/yr salary before bonuses. But they are in the same tax bracket. That doesn’t feel right to me.