Why don't we have a flat tax?

It would work. We have half of the united states not paying an income tax. HALF. So instead of giving that make more money 37% tax, which is ridiculous, why not give everyone a ten percent tax. We would make more money and people would make more money in the long run because business' would prosper, therefore people would make more money, and the economy would go through the roof. And people are saying, well thats not fair! a person who makes a million dollars a year will be paying the same as i do and i make 50,000 a year! Well it is fair the person who makes 50,000 will pay five thousand and the person who makes a million will paying one huundred thousand! SInce we have so many people the government would still make the same amount of money and maybe even more. And they tax us on everything else and get that money too! Why wont this work?