Because if an APK doesn't declare that permission, then it still can't access the Internet.

But why make it a "permission" then, if it is automatically granted? It should just be part of the regular API.

Probably some combination of inertia, future-proofing, and because it might facilitate some sort of internal bookkeeping inside the OS and/or the app store. After all there are quite a few more of those permissions around that have to be declared, but which are automatically granted and not specially surfaced within the UI.

Why would an APK want to access the internet ? (why my Phone app needs internet access ?)