The two big things here in my mind are:
1. Android Automotive has gotten very popular since it provides so much and writing your own OS is very very hard and expensive as so many car makers found out
2. Aftermarket head units often use it (see #1) so it’s likely far easier to get out there than if you had to compromise Ford/VW/Volvo/whoever
The actually vulnerable system is a custom vehicle head unit that merely happens to be running a software stack based on AOSP. It's not even "Android" in a product marketing sense.
Again, it's like blaming Debian because some loon stuffed it in a wifi NAS or whatever and put a backdoor into their UI. It's insane.