Is it really so surprising? Top institutions attract more PhD students and postdocs to apply, so better ones get recruited in average, and then they are put in environment of better PhD students and postdocs. There are great people at other institutions, too, they are just fewer and farther between.
I'm not 100% convinced it is just the recruiting better people, I do belive it is also something about being in a good environment. When I started my PhD noone jag graduated from that department in 8 years, but after my 4 PhDs graduated quickly. Having someone to compare you with makes a difference.
That is what I meant when I said that they are put in an environment of better (existing) students and postdocs.
But probably it is also essier to recruit good postdoc/students to a good environment. My colleagues in Kansas have real problems recruiting all the way from master students to faculty, while we have 100s of applicants for each position (not postdocs though)