Nicholas Lemann:

College costs so much because people are paying for unstated social goals. Each college and university is a collection of many different activities, some of which pay for others. Research libraries and philosophy departments can’t possibly make money; they require subsidies from business schools and biomedical-research labs, but that drives tuition higher than it would be if universities dropped their money-losing functions. More broadly, the United States created the world’s first mass system of higher education. Hundreds, not dozens, of colleges aspire to be research universities—because there lies status, prestige, and intellectual excitement—and so they have faculty members with low teaching loads. That costs money. The system is built to take in just about all high-school graduates, and that costs money, too.

I said that in the comment section of this post and in this article.