It is basically an arms race between search engines (specifically google) and SEO. Google does change their algorithms, but with enough effort (and money) SEO p

It is basically an arms race between search engines (specifically google) and SEO. Google does change their algorithms, but with enough effort (and money) SEO proffesionals can figure out how to tweak their sites to keep gaming the system. And the casualties of this war are the authentic sites that don't spend a ton of money on SEO.