First, Google doesn't openly disclose it's algorithm.  So we are left to reverse engineer it based on results.

Second, Google has in many cases manually intervened and changed search results.  They have stated this publicly.

Third, there are many other search engines other than Google!  Each has a different algorithm.

Fourth, many internet users are no longer searching Google.  They are searching Facebook, Youtube, Digg, and other sites.  Google and other search engines do not impact these sites in any way.

Fifth, there is a lot of information about SEO that is self-promotive of a particular product or service.  If you ask Google about SEO, they will suggest you purchase AdWords.

Sixth, Google is a business, a commercial company with a market cap of 172 Billion as of this writing.  They didn't create 172 Billion by providing results, they did it by selling ads.

Finally, SEO is constantly changing.  Google has publicly announced a major change in it's algorithm, but SEO professionals can attest that it is changing regularly.  Also, SEO strategies can become saturated once they become widespread.  What worked last year, or even last month, may be significantly less effective.