

As someone pointed out on the comment, one of the reason is Money, the other one is safety and security based on Vehicle manufacturer claim.
Yet, nobody can stop you to sniff and monitor your own car CAN/OBD2 interface using as simple as cheap microcontroller and compatible OBD2 interface kit (e.g CAN Transceiver/Controller kit).
EDIT: So far this curated list helpful to me for CAN bus based protocol.
Huh, surprisd to see SMT V is not make it to the list.