This really isn't that surprising. It could also be explained largely by self selection. If one believes that top schools by prestige also attract the top students, and that these schools are reasonable efficient at selecting among these applicants we'd expect a strongly one sided distribution of talent.
If the top 10 institutions have the vast majority of the top 1% of grad students, I'd expect they produce the vast majority of professors.
If the top 10 institutions have the vast majority of the top 1% of grad students, I'd expect they produce the vast majority of professors.