Because GM wants to charge subscriptions for all kinds of software features and don’t want any competition. They are mainly looking at this as their maps vs the phone’s maps. They are ignoring all of the other reasons on why people choose AndroidAuto and CarPlay. That is often as much about personalization as anything else.

Software features? Isn't GM primarily responsible for the OnStar network?

It seems to me that what they want to do is create an android-powered internal system that requires a subscription to their cellular service to use.

It's not like it stops people from just using their phone in a standalone way, which is what I do even with CarPlay. Bluetooth and I don't get along, and I was prompted to activate Siri and all kinds of other stuff that I have disabled on my phone when I tried it. No thanks, it works fine on its own.

And Android Automotive supports CarPlay.