And, it is actually too high to assess on the poor.
Gabelle - Wikipedia
Look, realistically you have to exclude an underclass from the tax somehow, unless you want to take the draconian step of culling your poor (Germany did that regularly in its HRE incarnation). The counterpoint is not true, 20% which is what we pay is not rebellion inducing for us, you're not going to pick up a gun and start shooting cops and the mayor over your property tax or income tax bill (and if you are willing to do that, I did not encourage it). 15% on extreme poverty was enough to regularly assassinate sheriffs of the UK, gendarmes of the French, and various other tax collectors and administrators during the medieval age well into the modern for unreasonable taxation. That's why we have both progressive taxation and the idea of land use taxes on the books, because it collects enough revenue both directly and indirectly without inciting rebellion.
Then, there's the other problem of collection. Is it worth an IRS auditor's time to go after a tax difference of less than $1000? No, because it costs the IRS more than that for a cursory audit (a line-by-line audit runs somewhere between $600-800 a day). You'd basically have a situation that if you imposed a flat tax, there would be a bunch of people who wouldn't care enough and the government could not collect on them anyway.
The entire point of taxation is to generate revenue for the government. You might not like what the government is up to, but unless you can marshal the votes to find representatives who aren't plutocrats, you're stuck with this system unless you want to leave. But freedom costs something, those M-16's don't just magically appear from the Springfield Armory in our possession. And we pay nowhere near the amount of taxes that less interventionalist nations pay that don't give their citizens anything.
I tend to take a Republican viewpoint toward taxation in the sense that most of our expenditures are frankly stupid. That said, I am under the belief that increasing the national debt is even more stupid, and that taxes should adjust upwards on everyone to account for it (and the US can actually go after transnationals by the German method, which is a taxation on transfers, but the US is always lobbied on both parties never to enact that particular feature). Taxes as they stand are too low now, and this tax bill goes in the wrong direction not because of tax relief, but because it adds to a national debt that default becomes an increasingly unavoidable prospect. If you are to maximize revenue, the taxes should be borne most by the parties who can be taxed without rebelling, the rich, and not to tax the parties most likely and most able to rebel, the poor and especially the veteran poor.
If I could tweak any particular law on revenue grounds alone and be completely uncharitable, I would basically repeal EMTALA, and let drug addicts, innocent babies, and other hopeless without insurance or means die without treatment like we used to before the Great Society times passed. Remove the safety net and let people die has an effective way of increasing productivity (by getting rid of your unproductive citizens). Healthcare was always meant to ensure the working populace stayed productive. If they aren't contributing, there's no reason for their access to healthcare as humanitarian grounds are precious enough already to give to workers.