It's not too hard to implement them with cryptographic protocols to prevent duplication and apply time/location restrictions to them. Moreover if you really wanted to steal a car, there are much easier ways of doing that, like buying a replica gun on aliexpress then going to your nearest intersection.
The problem is not really that they can get stolen, but remote control, like a bad person gaining access to the car to hit people or something like that.
Not every social problem needs a technical solution. You can steal cars now, this is solved foremost by most people not being thieves and then second the existence of police for the few people that are.
