We have progressive taxes because that is the way our Founder's wanted it. Progressive taxes were one means to prevent the
unnatural concentration of wealth in the hands of a few.
Some people hear "progressive" and have a pavlovian response.
Here is Thomas Jefferson (a favorite Founder of small government lovers everywhere):
Another means of silently lessening the inequality of property is to exempt all from taxation below a certain point, and to tax the higher portions of property in geometrical progression as they rise. Whenever there is in any country, uncultivated lands and unemployed poor, it is clear that the laws of property have been so far extended as to violate natural right.
Equality: Thomas Jefferson to James Madison