You're ignoring the power they have to shape minds during that 4 year period, which is particularly powerful for residential students who are out of the nest for the first time, how they can keep doubleplusungood people out of a lot of professions by not letting them graduate (it's now getting explicitly official in some, and personally sobering for me, the moderate conservative-libertarian I was in 1979? No way today I could become a scientist in the US today), and apropos of that, they control who gets to teach K-12 students, the Overton Window in K-12 curricula (which has had a serious allergy to effective teaching of reading for close to a century now, and much later math), etc.