It is obvious that serving ads creates misaligned incentives for search engines.

Sergey Brin and Lawrence Page (1998)

“Currently, the predominant business model for commercial search engines is advertising. The goals of the advertising business model do not always correspond to providing quality search to users…we expect that advertising funded search engines will be inherently biased towards the advertisers and away from the needs of the consumers…Furthermore, advertising income often provides an incentive to provide poor quality search results.”

There are tons of people whose sole job is to game their way to the top of Google, so it shouldn’t be surprising when search results deteriorate in quality.