But what does it mean that you are on a particular URL?
For a long time already it's not being connecter to a particular physical server. Now it's the next step - to be completely decoupled from the server and just mean content instead.
This is meant to offload tracking from just Google Analytics and SERP clicks, which is used to track user behavior (but can be blocked) into services that cannot be blocked beyond Google domains.
If Google hosts the website and is masking the resulting url, they're able to have more visibility than Google analytics. They'll likely give this AMP some SEO boost temporarily and that will get web admins to adopt the technology.
It's just like reCaptcha, which is used to track users across the web (requires google.com + gstatic.com urls to load, which drops its own cookies or scans existing ones), blocking recaptcha will break core web functionality... and recaptcha v3 is even worse.
Web publishers don't necessarily want their content decoupled from their own servers, but they don't have a choice now if they depend on traffic from Google.
You are not decoupled from the server. Google still sees HTTP request you make in plaintext and collects your data according to their privacy policy. It just won't be obvious because of publisher's URL in the address bar.
For a long time already it's not being connecter to a particular physical server. Now it's the next step - to be completely decoupled from the server and just mean content instead.