I'm not an expert but Google AMP is not a simple CDN cache. The Google-owned servers also proxies API calls to whitelisted urls like approved ad networks and Google Analytics. (Expand all the triangles in the list: https://amp.dev/support/faq/platform-and-vendor-partners/)

A cache service like Cloudflare CDN doesn't do that. (Although there's an example of somebody trying Cloudflare's new "Workers" functionality to proxy an api call on the server side instead of the browser side: https://community.cloudflare.com/t/google-analytics-proxy-wo...)

For publishers that use ad networks, Google's AMP is a different value proposition from a typical CDN. Because Google's AMP has a different technical architecture, it solves a different problem than a CDN.

Yes, Cloudflare could conceivably add more functionality to their service to proxy ad network api calls like Google AMP but it wouldn't buy them as much of a speed boost because Cloudflare doesn't own/control the "google.com" domain that serves search results. It's "www.google.com" that has Google's trusted javascript code to preload AMP data that is executed on the user's web browser.