Because the more money you print, the less valuable it becomes. We may be able to print enuogh money to give you and everyone $1 Million, but now the prices will adjust across the country and a loaf of bread will cost $3,000 or more. A new car will cost $15 Million. If you print too much to pay off the national debt, the money becomes worthless. But I should note, that this seems to be what the government is doing anyway.