That's true here.
1. The kids who graduated due to being smarter and more diligent would outpace their peers even if they hadn't attained a college degree.
2. The salaries of a minority of students who graduate from prestigious universities and/or from particular majors skews the mean.
3. It leaves out opportunity costs. Where could many of those students been if they hadn't spent 5/6 years in college at non-prestigious universities?
You and I have read a litany of articles on "underemployment" of college graduates. That begs the question: are those graduates actually underemployed? Most students are below-average students. I have the stat somewhere on how little today's college students spend studying. It's remarkably low and there's little evidence that this is because they know more than students in the past and can afford to not study. We have tens of thousands of college grads working non-managerial retail and service sector jobs. Many/most of those jobs could be had without college degrees. And, again, pretend that those smarter, more diligent peers had never graduated college. They would be outpacing the less intelligent, less diligent peers in those service sector jobs, as well.
There also isn't much reason to conclude that the bulk of college students today are emerging much more knowledgeable than when they entered, save for some of the fields that tend to attract the smartest kids, anyways.
Encouraging kids of average to below-average intelligence who aren't particularly good students to go to non-prestigious four year colleges because "they will earn $1 million more over their lifetimes" is usually not a responsible thing to do, knowing the debt most will accumulate and taking into account how many people like them leave college and have to accept jobs they could have gotten without spending five years in college.