flat-tax also increase inequalities:

 1000$ a month with a 20% tax: you already have     difficulty to pay for your food, goods, appartment
10000$ a month with a 20% tax: you           have no difficulty to live

In some countries, you have a progressive system:

poor : no tax
lower class: small percentage
midle : normal
high: high percentage

So everyone can live without being strangled.

Take a look at poverty rate in some liberal countries with the flat-tax.