Apple's Extensions operate over IPC. Trying to add overlays onto a live camera over IPC would probably introduce unacceptable lag. Especially if you also need to handle touch events too. And of course you wouldn't be able to actually do much besides have an overlay, e.g. transitioning to other screens, showing other UI controls etc. Plus this would significantly complicate the already-complicated Camera app interface.

And what would be the point? You can just launch a 3rd party AR app and it will use the camera APIs to show you the live camera feed, exactly like the built-in Camera app does.