That is, the current proximity-based keyless access would work the same, from the customer's POV: you're around and you have the key, the car grants you all access. You're away, and the car beeps and locks up.

It would just require somehow larger and more expensive components in the keyfob, which is anyway a rounding error compared to the price of the car. But, more importantly, it would require to make a fuss and change something in an area which "just works" now, which is always a hard and thankless task in a large corporation. Beside that, car manufacturers have little interest in making your car harder to steal (unless they look excessively bad compared to every competitor); they'll gladly sell you a new car instead.