You need to press buttons inside the car, buttons on the currently paired key (to prove possession of that) and buttons on the key you want to pair with.
So a passer by would have to press a button on their fob at just the right moment. Then when you go to test your new key fob, it wouldn't work, so you would pair again until it was your key that was paired.
Whatever the case, making it easier to pair, shouldn't be the primary focus, no need to help a thief doing it quickly. It would just be nice to have a way to do it, that didn't ultimately require the manufacturer to get involved; but that does remove a big hurdle for thieves, too.
There is CANBUS to the headlights, but that is not a OBD2 port. And more securely designed cars can put that in a less secure zone, so it can only send and receive commands for exterior things like lights, and not be able to have commands for keys injected, because that bus will not accept those commands.