I would argue that cars are not extremely risky to modify mechanically - the parts are usually large, almost everything you'd want to do on your car has youtube videos and forum threads with people doing the exact same procedure, and you have pretty immediate feedback if you really messed anything up/the car is inoperable.

I don't think that is the same for self-driving software if you were allowed to hack it. It is easy to test if your brakes work, I think it may be harder to test modified software with the huge number of interactions it must deal with.