I'd imagine the anti-tampering clause (it's a felony!) of the Clean Air Act is one reason.

The wiring for the CAN bus can be made to be accessible from the outside of the car with a bit of force, which means that's all a thief needs to do to steal your car. Yank a headlight out until the wiring harness comes with it, making the CAN bus accessible, install the car stealing modchip onto the bus, unlock the doors, get inside, start the engine, and drive off.

https://www.autoblog.com/amp/2023/04/18/vehicle-headlight-ca...

There is a new unece regulation, requiring strong temper protection, which means at least CAN authentication. https://unece.org/sustainable-development/press/un-regulatio...