Why? Google develops ad-supported software, they run their own ad placement service for it, and they allow others to use that service to run ads on their own sites. I think there's a bit of a conflict of interest between running the website ranking service and running an ad service that ranked websites can choose to use, but that's minor compared to also controlling the browsing platform.
If Google started ranking sites that use AdSense higher than other sites, you could probably prove that in court fairly easily — your lawyers would make them hand over their ranking code during discovery, and it would say `if (usesAdSense) score++`, and you'd win.
But if they made (are making) subtle web platform changes via their control of the browsing platform that hurt competitors and help Google, that's extremely difficult to prove. Look at third-party cookies, for example.
If Google started ranking sites that use AdSense higher than other sites, you could probably prove that in court fairly easily — your lawyers would make them hand over their ranking code during discovery, and it would say `if (usesAdSense) score++`, and you'd win.
But if they made (are making) subtle web platform changes via their control of the browsing platform that hurt competitors and help Google, that's extremely difficult to prove. Look at third-party cookies, for example.