The spec for amp specifically has an "amp-ad" tag for displaying ads. There are multiple ways they can be shown in an AMP page. The nice thing about AMP ads is they will not cause the page to change layout/flow as they load. I believe a size must be declared for ads to prevent bad behavior.
I didn't say they couldn't show ads, but basic ads aren't the only way that these places monetize. Trackers, 'you might also like' panels, etc all feed in to the overall monetization strategy. A couple of amp-approved ads on an amp-page don't necessarily make up for all that...
I'm not trying to avoid anything. As a user I don't even get a choice for AMP or no AMP. In fact, I changed my search engine on my phone to DuckDuckGo specifically to get rid of AMP pages.
https://www.ampproject.org/docs/reference/components/amp-ad