Because the autonomous car is a systematic issue that could affect every single car on the road using that manufacturer's software, and the "driver" of the car has no way to stop or mitigate the risk, or cause risk. He puts his life in the company's hands each time he drives the vehicle.
I'm not sure why people even embrace self-driving cars. By now we know that centralization, lack of maintenance, and fragmentation in software are serious risks, as well as how software can increase the attack vector on people as well as provide benefit. I'm not sure if the risks are worth the modest efficiency increase, and this isn't even getting into existential risks like external parties being able to control when you drive, or attacks on the networks or technology.