"While a flat tax imposes the same tax percentage on all individuals regardless of income, many see it as a regressive tax. A regressive tax is on which taxes high-income earners at a lower percentage of their income and low-wage earners at a higher rate of their income. The tax is seen as regressive due to a more significant portion of the total funds available to the low-income earner going to the tax expenditure. While the upper-income payer still pays the same percentage, they have enough income to offset this tax load."

https://www.investopedia.com/terms/f/flattax.asp