It isn’t, though, because of the marginal utility of money. Someone making $10k/yr sees much more value from an extra $10k than someone making $10M. For one it’s a lifeline and for one a rounding error. While a flat tax is fair in numbers it’s not fair in marginal value or utility. IMO that’s a much bigger deal. There are other macroeconomic advantages to giving the little guy the extra, though I don’t want to repeat myself so I’ll keep that in my sister reply.

FWIW we’ve already got regressive taxes in the form of payroll tax, which kick in from $0 and cap out once you earn enough. So unless you’re unemployed, you’ve paid taxes even if your marginal income tax rate is 0%.