Look. We have to pay for the stuff we use. We can either pay the government, or some other entity. It doesn't matter. Paying the government for roads, etc. is generally a good thing, because people don't want to be nickel-and-dimed every time they drive down the street. Large corporations in general don't want to be nickel-and-dimed for each segment of shipment they make. Police aren't privatized and we don't have "legal insurance" because that would be annoying. Education isn't privatized, because we like the idea of equal opportunity.
But we still have to pay for it all, and that's where taxes come in. In general, someone smart noticed that people who make more money use up proportionally more of the resources: they buy more things, so they use the roads disproportionately more; they have more things stolen from them, so they use the police disproportionately more, etc. Megacorporations do the same thing to an even greater extent. This is why we have a progressive tax. A "flat tax" would be completely unfair to the poor, as they are the ones who use the least amount of resources, and have the least means to pay for what they do use.
The total income of the US is somewhere around $6.8 trillion (via wikipedia, mean household income * number of households). 10% of this is not enough at our current spending rates.
However, all this is beside the point. The point is that we're taxing the thing we want to encourage: making money and spending it. Making money and spending it is what drives the economy, and we're discouraging it by taxing it. That's a bad thing. We shouldn't have an income tax, or a sales tax, or a consumption tax, or a transaction tax (although if we want to discourage HFT, that might be a thing to look at). We should have a savings tax. Of course, saving for retirement is different, but if you're just sitting on money for no reason at all, other than your fear of losing it, you're hurting the economy. This is the behavior we want to discourage, so we should tax the hell out of it. This has the benefit of the poor who make just enough to get by not paying much, if at all, and the rich-beyond-all-belief being taxed the hell out of IF AND ONLY IF they're not doing anything useful, largely making it a progressive tax, which is a Good Thing (tm). After all, if they're not going to do something useful with their money, why let them keep it?
That said, of course there are flaws in this system that need to be fleshed out, but it's at least something worth looking into.