Not a particularly difficult concept as to why Google would have a worse PR than, say, Twitter.
1) Google is the most popular search engine in the world and so doesn't to rank for any particular keywords. You search the keywords on Google, not search for Google using keywords!
2) People don't link to Google anywhere near as much as they link to Twitter. You tend to link to the direct source, not the search engine!
3) Their site isn't built with SEO particularly in mind - quite simply, because as noted before, it doesn't need to be!