If government is a service, everyone should pay the same amount, as in a restaurant. Of course, as a practical matter, this would be impossible in 21st century America. To spread the extraordinary cost of government equally to each citizen would mean taxing millions of low-income people into destitution. Those who benefit the most from being part of this society would pay the most. The idea of charging everyone the same percentage is not really that radical.
Sales taxes work that way.
So do Social Security taxes. From time to time, people such as the flat-taxers suggest we do this with income tax, but the idea never really catches on. Too many constituencies worry they have too much to lose, and government can't do anything so sweeping, anyway. Also, many people have come to view a progressive tax system as the only fair one. In our system, a wealthier taxpayer pays a higher percentage of his or her larger income.