Google's algorithms are proprietary, and they have data from tons of sources beyond links (including clicks) that they could be using to guide search relevance. How do you know that they're not?
I don't think number of clickthroughs is considered because of how easy it would be to game.
EDIT: In fact, I recall a guy using MTurk to test this theory. IIRC, He had two similar pages and was having people execute search queries and click on his site to see whether its position would improve as a result of it. Unfortunately, I don't know whether he shared the results.
EDIT: In fact, I recall a guy using MTurk to test this theory. IIRC, He had two similar pages and was having people execute search queries and click on his site to see whether its position would improve as a result of it. Unfortunately, I don't know whether he shared the results.