Many phenomena where a product that was once considered standard or relatively cheap becomes expensive are studied under the name of "Baumol's Cost Disease". This does not apply exclusively to college in the United States, but to many sectors and many countries. The root of the phenomenon: when we say "college is more expensive" we mean "the amount of consumer goods I could have purchased with the money it costs to attend a seminar has gotten larger". We have gotten much better at manufacturing goods, but teaching seminars is still just as inefficient.

This is easier to understand if you think about maids, cooks, and butlers. It was always true that to have a servant, you had to match the wage that person would earn in a factory (with modifications for perks, difficulty, independence). As the industrial revolution went on, the middle class got very wealthy, so you would assume they would be able to hire more servants, right? Wrong; the more productive the factories got, the higher wages rose, and the fewer families felt they could afford to buy a servant.