The biggest perverse incentive for Google is that making better search results can mean less clicks to ads This gets close to the real root of the issue -- att

The biggest perverse incentive for Google is that making better search results can mean less clicks to ads This gets close to the real root of the issue -- attention is monetizable independently of the quality of content. There would be much less incentive to create SEO spam if search engines negatively weighted pages with ads and affiliate links, and if manufacturers were barred (e.g. by the FTC) from owning or imitating reviewers.