Then the thief could hide in the back, and rob you while you were driving down the interstate (ie. he could hold a knife to your throat and force you to drive to location that was hidden from public view). I don't know how realistic this is, but it is far more dire.
If the objective is to steal your car, the thief is going to use the least risky way that he can come up with. If you take away his ability to do it without having you start the engine yourself, this is what he's left with.
