The point is that, because it's a flat rate, you end up paying more the more money you have.

The rich dont spend much of their money on consumer goods. They spend most of their money on investments, financial services, etc.

So when you and I spend nearly 100% of our money on consumer goods, we are paying 25% of our income in taxes. But the richest among us, whose consumer spending amounts to 10% (or less) of their earnings, pays just 2.5% of their income on taxes.

Flat rate taxes on consumer spending are wildly regressive.