[dwaite]

> I think the extension could communication with its containing app, but the system is not set up to do that.

and on iOS, the containing app is likely not running (in a GUI context).

Typically, the extension is its own small program bundled within the app. The app can be run by an end user, while the extension can only be invoked via the extension mechanism.

[mattiemass]

Same thing on macOS. The containing app does not need to be running for the extensions to be active. You are right that the extension bundle can only be used by this mechanism, but interestingly the containing app can use its own extensions (though this may not make sense in many cases).