Because the car has been the only place a radio might be located for the last few decades not because it was a great option in and of itself. Wanting to mandate emergency radios be made available to every person is a matter in and of itself. Even implementing distribution as part of getting a new car is a separate topic. Requiring it be built in to the structure of the car is neither a good option for safety or a good option for efficiency. The only reason we're even discussing it is it was the last place to use it normally.