They make the tax code complicated so they can leverage it.
This is why they can't get Trump on tax issues. Trump pays the best tax people to cover him. If they nailed Trump on anything they'd have to nail a lot of other people too, who are otherwise untouchable.
A flat tax would work just fine, except for all the people in the tax industry that is. You just level the flat tax until you get the equivalent revenue from the current scheme.
There's no need for wagies with low net worths to go through the full process.