This seems like a bit of an over-simplification.. A car needs to be type approved to run on our streets and the insurance companies need to accept whatever you did to the car in case of a crash. You don't really want to end up paying 100% yourself when you crash into that school bus with your mod.
However it would be interesting to learn how type approvals handle the self-driving things. Maybe they really do allow all kinds of computer-assisted things? I don't know. If you change some structural beams in the car you surely lose approval. But can you mod steering and braking? What about openpilot users that crash?