maybe because it's not so hard to use "site:..." on google.
Using site: on google is a terrible alternative to search on YC. Google's ranking algorithms might work for the web at large but it doesn't work so well for searching HN. SearchYC does a pretty good job at searching YCombinator and I would like for such a feature to be integrated into this site, but I'm guessing PG has better things to do with his time.
Also, searching via google doesn't always work well because there are redundant copies of most comments on both the relevant post and the commenter's thread page.
I still think a "Search" link at the bottom of the front page (or a note in the FAQ) would be good, whether it points to searchyc.com, "google site:news.ycombinator.com", etc. People probably only need to be told how to search once, but searching for how to search is a catch-22.
That's absolutely horrible, since Google indexes individual replies here and often times you end up clicking 10 links that all belong to one post.