J. Bradford DeLong is asking a very valid question: considering that the difference between the earnings of those with a college education and those without is approaching fifty(!) percent, why in the world is the growth of education going down?
It's a valid question, but I think it has a socio-cultural answer, not an economic answer. People are stuffed full of stories of people without college educations or who dropped out of college (like Bill Gates) who have become filthy rich, and stories about people with Ph.Ds who are driving cabs. The fact that these sorts of stories are notable precisely because they seriously buck the trends is lost if one doesn't already know that those trends exist, just as the increasing visibility of violent or spectacular crime often causes the public to feel less safe, even when (oft-ignored) statistics show that crime rates are going down.
Then again, Brad's insistence that "every American who can should go to college" contains one incredibly big assumption there.