As I've already been downvoted for this, I'll explain: the lower your income, the greater the proportion is spent on straightforward living expenses, generally.

As I've already been downvoted for this, I'll explain: the lower your income, the greater the proportion is spent on straightforward living expenses, generally. If you earn $10k/year, all of that will be going on basic necessities, so $1,500 a year in income tax (15%) is $1,500 straight out of your daily/monthly budget paying rent, food, transportation, etc. The higher you go in income, the greater the proportion goes to things like retirement, investments, entertainment, etc. Millionaires (i.e., earning > $1M/year) paying 15% don't worry about eating well or enough, or being cold at night or losing their job because their car breaks down, or even whether they can pay for college for their kids. This is why flat taxes are regressive, and if you propose a bottom line exemption (e.g., < $20k/year pays no tax, or less tax), it's not a flat tax anymore, it's a progressive tax, and we're just haggling over the details.