Except for the car already doesn't allow you to lock the doors with a fob in the car unless you use the other fob, so it also uses proximity... The car already has the pieces and logic to do it.. a simple programming would enable it easily.I agree strongly with your last statement.
I don’t like to be pedantic, but please bear with me!
The current feature that has the car lock the doors when you walk away without opening a door is based on time.
The auto lock feature that would kick in when you walk away even if you have opened a door would be based on proximity.
Those aren’t really the same thing, so they don’t already have the feature on the car.