[tsotha]

>The best possible tax scheme for poor people would be a simple flat tax that is impossible for rich people to avoid.

You're looking at the wrong end of the telescope. It's really not that difficult to look at a tax table and cross-reference your income, so a flat tax is a very minor improvement.

Where every tax system gets tripped up is in determining what qualifies as income. People who manage to avoid taxes aren't somehow lowering their rates; they're getting inflows of money excluded from the "income" accounting box.

It's not an easy problem to deal with, either - states have been trying for centuries. Most of the write-offs individuals and corporations take are perfectly reasonable for some situations and very much abused in others. As you try to prevent abuse you end up with a tax code that's so complicated there are more ambiguous "gray areas" for people to take advantage.