Because they can afford it. It's a redistribution tactic. You can also phrase it like this: college should be free for all to attend. Then, as long as you have a progressive tax scheme, the outcome is the same. Cheap for the poor, expensive for the rich.

According to the old story, the New York Times asked a famous bank robber why he robbed banks. The answer: Because that's where the money is.

The money for funding public and quasi-private (universities and hospitals) institutions has to come from somewhere. Making it equally affordable for everybody doesn't raise enough money to maintain operations. Same for funding the government.

Granted, I think all of those institutions are due for reforms, which have little chance of happening right now, but still, I think the basic funding equation can't be eliminated.