So the question arises, why the search engines guard their algorithms so closely. The answer is quite simple - they value the user's experience. Their aim is to display the most relevant results according to the searched query to satiate the searcher's thirst. If the search engines publish a guide stating all the factors affecting the ranking process, all webmasters will implement them and the displayed results will be easy to manipulate. This will defeat the relevancy factor as well as deteriorate the users' experience.
The inherent dynamism of the algorithms has its own justifiable reasons. Over the time, webmasters with the hit and trial methods decipher the factors affecting the rankings. Moreover, these unfolded secrets are publicized and hence known by everyone. This in turn instigates search engines to bring slight changes which will uphold their aim of relevancy.
So, it is important to strike balance between seeking the eternally unknowable search engine algorithm and making sure that the SEO efforts are directed in the right direction. Many well known search evangelists still promulgate:
1. Good HTML titles and Meta description
2. Impressive content
3. Eroding any kind of roadblocks to crawling
4. Building quality backlinks