They could advertise support for the Bose/Sony/Apple/whatever premium entertainment system; a standard - even a very basic one - might also reduce development work, as you can use preexisting connectors etc.

And, at least for me, if I would buy a new car, repairability and replaceability or parts is a consideration. And if I had $1000 to spare, I'd buy a hifi unit from a dedicated manufacturer - and would prefer a car vendor that makes that easy. I've never heard the entertainment system be the decisive factor in choosing a car - but I know people who put off buying an e-mobile because they usually have annoying computers built in.

Finally, I don't think the car vendors would neccessarily do this. You'd probably have third party vendors selling adapters. Crazy cable tree in, nice plug out.