First, what happens if the electrical characteristics of your vehicle change in some way? New vs old battery? Busted headlight? Phone plugged into an outlet? Diesel air intake heater grid kicking in on a cold day? What if you need to jumpstart your vehicle? It just seems so finicky in the real world.
Second, what's the point of using this analog signaling system to begin with? I don't see the supposed simplicity of it. Both the transmitter and the receiver are more complex than would be needed for digital. The other argument is that it is somehow more "hacker-proof", but using analog signals doesn't make it so. You can have a similar scheme operating on the CAN bus with no added risk. In fact, I bet there are devices on the CAN bus that can both measure and modulate battery drain, so the isolation may be illusory.
Ultimately, it's not about not having the technology. It's just that your average customer favors convenience features over having a fortress on wheels. Plus, the returns on sophisticated defenses are diminishing, given that a car can always be loaded onto a tow truck, the hood can be popped open, or the whole thing can be stripped for parts with a Sawzall (as catalytic converter thieves tend to do).