system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why wouldn't a flat tax work for the United States?

Like Herman Cain had proposed with his 9/9/9 stance. A flat tax for income, state, and federal.
gpt
The reason is that it would actually increase taxes on the poor.  
Sales taxes are the same for rich people and poor people, but they hit poor people harder.  
The reason is this:  
  
Let's say the sales tax is 10%  
Suppose you are a rich person making $1,000,000 a month.  
There is no way you are going to spend that much money in the month.
Maybe you spend $200,000 that month and save the rest.  
That mean the sales tax is $20,000 or 2% of your income.  
  
Suppose you are a poor person making $1,000 a month.  
When your pay check comes in your are likely to spend almost all of it, on rent, bills or food. Many do not have left over to save.
Say you spend all $950, you pay $95 in taxes
That means the sales tax is 9.5% of your income.  
  
9.5% is a lot bigger than 2%, which is why income tax seeks to equalize it.  

As to why it makes sense economically. Again you give a rich people a dollar they'll save most of it, maybe they'll invest a small part of it. Give a poor person a dollar they'll immediately put it back into the economy.