[judge2020]

A large factor is whether or not there are any contextual ads at all for the content being viewed. Sure, random sites with amazon product lists might benefit from letting Google Ads scrape their page then show ads related to the products, but random forums and game sites might not have any contextual ads that would sell.

For example, dotapicker.com (which allows you to see counter-pick viability in Dota 2) runs ads but they're either ads of web-based video games that shows a lot of skin and have very vague wording to skirt Google Ads guidelines (about a quarter of the time), or random product and services related to my overall browsing history, eg. Amazon/Etsy/Ebay/etc or B2B services like Monday.com.

Contextual ads definitely work where they can, which is why google.com ads are all contextual to your search, but it expands available ad real-estate to run targeted ads in places that otherwise would get no clicks.