Just because metal conducts electricity, doesn't mean the source of electricity would be enough to kill him. The fact that these things aren't in cars already r because, 1) because having a big conduction plate would originally be faulty, like all things r, which might potentially be very harmful, 2) If someone is going to jack your license plate, they may do it reguardless because electricity is easy to get around and 3) The cost of recharging the plate from all the times it would have to be running could be absurd.

Check local laws. A lot of states don't allow covers over the plate. And this would take some safety checking. If someone were to grab it during a rain storm, the next knock on the door may be a sopena for someone getting zapped.