High end home audio is a niche market. Cars are not. Any audio company looking for additional and compatible revenue streams can create brand awareness and potential future customers by inserting a speaker set or head unit into a luxury or halo car. The car manufacturer then gets to tout how great their "14 speaker XYZ Audio System" sounds, regardless of fact.
These "home audio" producers are all in on car audio so there must be value in it for them:
Meridian
B&O
Revel
Focal
Burmester
Bose (I know...)
Mark Levinson
McIntosh
Heck, in the Clarion ownership days, you could get a McIntosh branded Clarion head unit in a Subaru.
I have to say the stock Fender system in my VW is not the worst thing I've heard in car audio.