Except this isn’t true. A larger fraction of the population than ever before are getting a college education: https://www.statista.com/statistics/184272/educational-attai.... More Americans get a college degree than in most other developed countries.

What you’re seeing is an entirely different effect, which is that academic hiring focuses intensely on pedigree. Professors are drawn from the ranks of a handful of elite schools: Harvard, Yale, etc. Graduates of those schools are much more likely to be elites. For example, the vast majority of law school professors went to Harvard or Yale.