I'm self employed and had to explain to my kids why taxes in the US were so complicated and took hours to do over the weekend. There must be powerful forces at work because Republicans have touted a flat tax or a return the size of a postcard for decades but when they controlled all three branches of government couldn't get anywhere near enough votes to pass it. It's truly an abysmal system and I hope my kids don't h…
They don't actually try to pass a flat tax because it is not feasible to actually do a flat tax in a country like they US with such a wide range of incomes. Even with a low rate, there will be people poor enough that paying the tax is a major hardship. So every even remotely serious "flat" tax proposal has some cutoff and only applies to income above that cutoff.
But then it is not actually a flat tax. It is a progressive tax with two brackets. Once you get there, it is really hard to come up with a convincing argument that two brackets is better than more brackets.
If the calculation wouldn't mystify most taxpayers, what would probably be the most sound theoretically would be a continuous bracket structure.
The other reason they don't do it is that the flatness of the rate structure has pretty much nothing whatsoever to do with the complexity of a tax return. If you changed the rate structure from a progressive rate with several brackets to an actual flat rate, or a more realistic two bracket progressive system, that would shave about 1/4 of a page off the several thousand pages of the tax code and tax regulations.
The complexity of it is all in figuring out which income is taxable. Once you've got all the income classified into the various relevant tax categories, the actual calculations where you use the rate structure is trivial.