It is part of an incintive based system. If I know I am only going to make 200k a year, and the goverment will prohibit me from making any more, what is my reason to further myself or try harder when I reach that 200K cap?
If I start a business, and it does well, I should be able to reap the rewards of what my business makes, that is my incintive for taking on the risk of starting that business. Without the incintive, no one will try any harder then they need to in order to meet the minumum standard.