because of user expectations towards popular Apps. microG is there to make that big pool work that rely on some of the components for message passing, app accounts or device registration.
You can disable the communication. By default it is enabled to cause (in my view) the least amount of surprise for new users.
It’s a discussion to disable this compatibility layer by default, the user would need to be educated in 1-2 sentences what microG is about during the setup wizard and its implications.