Another thing I'd forgotten about; an increasingly common trend with top-end vehicles (not cheap pieces of shit as you claim) is integrating even MORE features from the CAN bus into the head unit, particularly climate control. This is becoming increasingly (and annoyingly) commonplace, and is starting to filter down into midrange vehicles as well.
It's only the cheap pieces of shit and high-end vehicles from a handful of makes which only hand-build cars (Koeningsegg, Spyker, etc.) where volume is too low to justify highly integrated units where you can swap a head unit and not have to jump through hoops to not lose any functionality.
Sure, in most vehicles you can either install a CAN hub or even a passive connector and get the car to run, but you will lose some of the original features and kill trade-in/resale value in the process - and for the vehicles which have jumped on the touch-screen-for-everything trend, good luck selling a car where heat/defrost/AC doesn't work.