Yes, they must depend on what you earn.. How can you ask a citizen earning 10 to pay 5 and also ask another citizen who earns 5.5 to pay the same amount. What about a person unemployed? How can he pay 5? That's the principle of progressive taxation, the one earning 10 will pay more than the one earning 5, so that the unemployed can still receive some services even if he cannot pay taxes.
In other words, "In America, we punish success." And then we wonder why there aren't any jobs...
A flat tax rate would force government to tax the poorest of people at a rate which they can still live reasonably well; this would decrease taxation by a huge amount, and force government to be smart about where they allocate tax money (such as what we pay them to do, like roads and bridges and a REASONABLE military to STOP INVASIONS, not big enough wage war against every other nation and blow the planet to smithereens.)
Instead, we throw more money at government, beg them to help us and to tax the rich to shit until they leave (and take their capital, and jobs, with them, to other nations), and cry that we can't support ourselves and need welfare as a staple of our lives, since we threw more of our money at government, and begged them to help us,...