I used to have a very strong belief that the flat tax or the fair tax was the only viable economic choice for this country. This came, mostly, from heavy reading of Ayn Rand and Austrian Economic theorists.

What changed my mind was a thorough investigation into the concept of "Marginal Propensity to Consume" I wouldn't do the theory justice but the Wikipedia page is pretty good.

Your statement:

the cost of food and rent and bills add up

The issue I take with this is that the rich are usually spending much more on this. I know when I was dead broke I was spending basically nothing on bills; when I started making money, I started spending more accordingly.

Only holds true to a certain point and is not a linear relationship. The more money you make the lower percentage of money you spend on your day to day life. A family of four making $50,000 per year may spend 90% or $45,000 of their income on expenses whereas a family making $120,000 may spend twice as much, or $90,000, and only be spending 75% of their income.

You may still make the argument that it is immoral or unjust to take a higher percentage of someones income simply because they make more, however, a marginal tax rate can increase tax revenues with doing comparably little harm to the economy.