How can anyone say with a straight face that a progressive tax is more fair than a flat tax? All else being equal. (If it's not, then that is what we should focus on fixing)
Easy, when you include the costs of living as well. Someone making $100k a year has a lot less money after rent/housing tax and food than someone making $1m, even if the first person pays 20% tax and the second person pays 40% tax. A roof over your head and food on the table is not an optional expense or some kind of luxury that you can forego when times are lean.
And despite what some people here on /. seem to think, the US is not willing to let its people starve to death in the streets (at least not en masse), so with a flat tax you would simply have more people depending on welfare, meaning the flat tax rate has to be made higher to pay for that ... Spread out over the entire population, you'd have a group that owes more in taxes than they have left after food and rent is payed.
Net effect: people who earn more money would pay more than the original percentage in tax anyway, so why not use a progressive tax rate from the start and at the same time have fewer people dependent on the government for their daily survival?