Exceptional people go to these schools. That's why the high achievers tend to come from those schools.
Yes and no. I would imagine that if you could define "exceptional people" empirically and then correlate "high achiever" to them and the schools they attend, you would still get a disproportionate number of people from the "elite" schools such as Harvard and Princeton (as a Harvard grad, I balk at the inclusion of Yale on principle).
These days, especially with a weak job market, who you know is just as important (or in many cases, more important) than who you are -- at least for getting your foot in the door. As people at these "elite" schools are already in a disproportionate number of high-level jobs, you are more likely to have a contact that will get you in the door.
I've seen this first hand. I had a friend that went to a state school for college -- he was very much my equal or superior (and his resume reflected that), yet because my resume had "Harvard" on it and his didn't, there were a few cases where I got called for an interview but he did not.