Head units are extremely restricted in what they can do with CANBUS, generally it's a read-only arrangement so the head unit can lock out functions when the car

Head units are extremely restricted in what they can do with CANBUS, generally it's a read-only arrangement so the head unit can lock out functions when the car is in motion, or show statistics. An example, in a modern car I recently purchased, there are settings in the head unit for a bunch of personalization aspects of the head unit... To change settings on the ADAS behavior, you have to use buttons on the wheel to control a menu that appears directly above the steering wheel: The entertainment unit has no ability to control settings for vehicle motion-related behaviors, so you have to deal with a less friendly menu that isn't touch capable to set them. This is also why Tesla is particularly unique (reckless?) in pushing OTA updates which can change the car's driving behavior. Nobody else would use OTAs that way, and most OTA-capable hardware in cars can't actually interact with driving behavior components.