Why do tax brackets work this way why is it that a Rich/Wealthy person the first few 000 dollars may be taxed?

Why do tax brackets work this way why is it that a Rich/Wealthy person the first few 000 dollars may be taxed at 10% than the next 30 000 dollars is taxed at 20% than over 3 million dollars is taxed may be taxed at 70% why does a Progressive Tax System work this why why can't the rich pay 70% on the first few 000 dollars too is it because than the Middle Class would be paying 70% too or am I wrong about this flat rate please help me understand ?