It's not always about that. AA and Car play are a very common reason for service visits and it's not uncommon for customers to get their entire head unit replaced because of frozen screens or whatever. GM hired a lead dev from CarPlay to help lead their replacement. When they work it's great but my touchscreen freezes regularly and it's extremely annoying.
So basically, automakers can't make a robust console to simply display the streamed content. What makes you think that their software will be any better?
It’s the natural result when software (and sometimes hardware) is treated as a line item to be contracted out to the lowest bidder. Infotainment is a core feature that shouldn’t be cheaped out on, but many automakers haven’t realized that.
It’s also just more lucrative to drive people to use the built in stuff instead of mirroring their phone, both for easier data mining and for pushing users into subscriptions.
Idiots.