Google has made it clear many times already, they want to have Android everywhere. And it appears that Android Auto isn't enough Android in the car. Google wants to be the backbone of the infotainment system. Not just a "plug and play" option that is built on top of many of the car makers systems already. While we've had Android Auto for a little while now, and we've tried it with a few different cars, it's not a complete infotainment system. Google wants to power your entire infotainment system with Android. And some references in the Android Compatibility Definition Document or CDD, seem to back that claim up.
If Google does indeed make their own infotainment system, one that's more robust than Android Auto, which is said to be called "Android Automotive" right now. It's going to be a bit tough to sell this to car makers. There are already some car makers out there who haven't gotten into Android Auto because they don't want to give all that info to Google. Of those auto makers are Porsche, Ford and Toyota. While Android Automotive would be great for Google's own self-driving cars, I have a hard time believing that this is just for their own cars, and not for other cars on the road right now.
It's quite clear that Google has a heavy interest in cars. After all they did just hire a key executive that spent time at Hyundai, to head up their self-driving car project. And creating an infotainment system will generate a lot of data to go back to Google to help them target ads, improve their services and build their self-driving car, obviously.
Now it's important to remind you that stuff found in the Compatibility Definition Document are not 100% true. This is a document meant for Google and their partners, and things are libel to change as well as mean something totally different than what we are reading into. So it's important to take this with a grain of salt and as a rumor. As it could be true, but it could also be false, and be meant for something totally different.