This is not what these studies measure, but people in Academia are much more likely to have gone to elite grad schools (in their respective fields, which is not the same as undergrad rankings of course). And, in my experience (Stanford undergrad, MIT PhD, currently UChicago postdoc), elite grad schools predominantly select people from elite undergrad institutions. It's possible to get into a great grad program from a non-elite school, but it's much harder, while someone like me in the middle of my class at Stanford was able to get into good grad schools without too much trouble.

This is probably not just due to snobbery; it's much more likely for the admissions committee to have heard of your recommenders and such. If you do research with a famous professor at a non-elite school you'd probably have similar results, but the barrier is a bit higher.