Yeah, and academia is fedudal fiefdom anyway, so this doesn't follow. Firstly, going to graduate school doesn't mean you'll become a professor. Peole from manual labor backgrounds will go on to grad school, they just won't go on to stay in academia: they will take better paying positions in private industry. Others will end up working for the government.
In the academic fiefdom, moreoever, it's always been the general rule that, if you go to the top three, then you'll teach at the top three. They tend not to hire outside of that incestuous circle and, when they do, they hire from the most prestitgious departments, few of which are located in Podunk State Uni.
If you went to Harvard, Yale, Princeton, it's a mark of dishonor to have to go to a second tier isnsitution. But, it's a market and, by definition, the jobs are scarce. Thus, some people will go to the next tier.
Similarly, those who attend second tier institutions can pretty much expect to work at the third tier universities. A lucky few will be hired within their tier -- but not many. (And, of course, sometimes a top-rated department will be in a school in the third tier, and a top-rated department well-known for a specific focus can be in a not-so-top-rated university, too. So....)
The feudal fiefdom was the norm through the 70s or so. When the job market started tightening up in the mid-70s, worsening through the 89s and 90s (esp in humanities and in some social sciences) people from Yale were applying to podunk places like the State University of NY @ Oswego and Cortland -- and they were landing the jobs.
Back in the 60s, by contrast, one of my profs, who'd graduated from Yale, took a position at a SUNY campus. The people at Yale were *horrified*. Hal had to take the job, though, because he needed to work (wife, kids, not from a family who could hlep him with the bills, but went to grad school on the GI bill.) That's another factor that has to be taken into account when you look at the numbers: a number of people from blue collar backgrounds did so on the GI bill.
Anyway, the glut in the humanities and social sciences markets really changed things. You'd hear the same stories over and over again. When you have 400-600 people competing for a slot, the hiring committee is naturally going to pick people from the best schools. After all, the structure of academia pretty much forces them to do so. Deans rank the departments on the basis of the pedigrees of their faculty.
This has also changed the structure of the schools lower down the tier as more and more people people from research-oriented unis populate Podunk State, they force everyone else to follow suit. That along with the increasing pressures on peple to publish at institutions where you used to be able to get away without publishing a great deal -- community colleges, commuter campuses, etc.
"We live under the Confederacy. We're a podunk bunch of swaggering pious hicks."
--Bruce Sterling