Because they're the ones who can afford them.

I've read several stories of colleges and universities actively recruiting foreign students to make up for the lack of local students who are finding it increasingly difficult to pay for the ever increasing (much faster than inflation) costs.

They don't seem to take the hint of LOWERING THE COSTS because they can tap these international markets to keep their inflated salaries for professors who work 3 days a week with several months off per year.