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.

Hmm… If we want to stretch something further… How do I know water that I drink is something new or clean instead of treated water, pure water or even perhaps dinosaur piss that has been filtered from underground soil long long time ago? 🤔😳