> Intra-computer buses like PCI get by without that.
and how much security do you have on a PCI bus protecting you from a rouge card?
(although, PCI actually does have a slot ID, so the main system can do some source validation. This is actually used with some virtualization systems, but answering the question in the spirit asked, rather than being picky about the particular bus used as an example)
the problem is that the CAN bus is not within a computer, it's connecting many different computers together to form the car's overall network. Not all of the devices on the network should be equally trusted.