I mean, if you make 10k a year your fixed costs leave less money for taxes than if you make 100k or 1m a year, so doing it on a simple percentage of gross incom

I mean, if you make 10k a year your fixed costs leave less money for taxes than if you make 100k or 1m a year, so doing it on a simple percentage of gross income (rather than gross revenue) is likely to accelerate poverty, which is very much picking winners and losers. Anyway the reason why we have deductions and tax breaks as such a big component is in large part because it is one of the things you don't need 60 senators to pass.