[Disclosure: Google Search SWE; opinions and thoughts are my own and do not represent those of my employer]
Why do you assume malicious intent?
The balance between search ranking (Google) and search optimization (third-party sites) is an adversarial, dynamic game played between two sides with inverse incentives, taking place on an economic field (i.e. limited resources). There is no perfect solution; there’s only an evolutionary act-react cycle.
Do you think content spammers spend more or less resources (people, time, money) than Google’s revenue? So then the problem becomes how do you win a battle with orders of magnitude less people, time, and money? Leverage, i.e., engineering. You try your best and watch the scoreboard.
Some people think Google is doing a great job; some think we couldn’t be any worse. The truth probably lies across a spectrum in the middle. So it goes with a globally consumed product.
Also, note, Ads and Search operate completely independent. There’s no signals going from Ads to Search, or vice versa, to inform rankings; Search can’t even touch a lot of the Ads data, and Ads can’t touch Search data. Which makes your theory misinformed.