Well, it sounds like you are unfamiliar with what economists refer to as utility and surplus value.
But as you acknowledge, there is never equality in use/benefit from government services. My claim is the rich benefit far greater because of the protection government affords them--the direct type: police, fire, food purity, etc; and hidden types: barriers to direct competition through licensing, tariffs, regulation, etc--gives them more surplus value than poorer persons (more utility per dollar). In my opinion, graduating the tax rates more closely matches the benefit received/distributed (surplus value) to the tax (price) paid than does a flat tax system.
I don't expect you to agree.