Well, because the implementation uses google urls, right? I assume they could have asked users to create amp.theirdomain.com CNAME records and upload appropriate certificates, right? Or the solution that cloudflare is using, or?
Ahh, I finally get it. An intentional walled garden that's a little bit open for now. I'm in the pot, and it's not terrible yet, but you just barely turned on the heat.
I assume it ends with everything in the serps being preloaded. And the only things in the serps being compliant content.
That language is misleading: only AMP articles are eligible for "Top Stories" placement, which means non-AMP results are completely excluded from the most prominent ranking on SERPs.
AMP may not be used as a ranking factor within each class of results, but it absolutely creates two distinct classes of results, where AMP is given priority over the open Web.
You're right in that I really shouldn't be directing that at you. I believe that you, personally, are trying to do the right thing. Apologies for the snark.
However, I believe the Google devs behind Froogle also had good intentions. They provided a framework to make product pages easier to consume for Google. They provided a real incentive for publishers to conform to that standard. They got nice placement in a carousel, cached display of product images, and so forth. Then, later, well...
Edit: And whatever the technical reason for the google urls, it does open up possibilities for the future that aren't desirable. It's a dangerous precedent.