Either you pay with your data (Google) or upfront for the phone (Apple).

I use Organic Maps on GrapheneOS and add businesses I want which aren't there (hence I had to look them up in Google Maps in Firefox Focus, which is painful enough that I don't want to have to repeat it).

You pay for cell service on your phone, which lets you access things like Google/Apple maps, spotify streaming, etc.

GM wants to charge for a separate internet service tied to your car, that still goes over the cellular networks, in order to access these exact same features in their head unit. It is completely redundant. CarPlay already offers a near perfect experience in this regard, without charging an extra monthly fee.