[dTal]

Because now you have the worst of both worlds. The computer still has the final say in what the car does, but now you have a fallible human in the loop at all times as well. If the computer really can predict crashes better than the human, then have the computer drive the car. If it can't, then letting it override the human isn't a good idea.

Example: you slam the wheel to the right to steer into a guard rail. The computer overrides you and you remain in your lane. You flatten the toddler you were trying to avoid, that the computer didn't see.