I think people are uncomfortable with the idea of Google controlling every aspect of your website down to the presentation. It'd be better for this to be centralized and standardized independently of Google's tendrils.

Part of what makes AMP fast is the preloading, so any AMP competitor would need to tackle that problem. You'd also need to demonstrate that websites rank better if they use your solution. And they should, if Google plays fair. If not, then at least this would be a public demo of that fact.

It's that and Google's CDN. If you hosted it yourself Google wouldn't include you in Search results carousel.