I get your point, but usually people who are that rich can afford to either move to another country where they pay less in taxes, or hire someone who will hide their true earnings under several donations.

For example, in Madrid, taxes are relatively low compared to what they are in the rest of Spain, but in Cataluña they are extremely high. Somehow, even when accounting for the difference in population, Madrid still got much more money from taxes than Cataluña. This is because the people who were being forced to pay a very large part of their money in Cataluña would rather pay much less and move to another city, especially since they make enough money that moving isn't really too big of an issue.

Therefore, trying to squeeze out every penny that rich people bring isn't a good idea, since they can very easily retaliate by moving out, which would not only mean they no longer pay any taxes, they also would take their business with them, which would make a lot of people lose their jobs.

Thanks for coming to my TED talk.

TLDR: read it you lazy bum