Great concept. Why hasn't anybody done this yet?

I'll get to it in my case but really this is on the providers; they designed the protocol backwards. Instead of each customer hacking in a PBX it would be trivial to ping the last known area and then if the phone didn't respond in n (customer configurable) sec send a broader incoming call request ultimately resulting in an all areas ping, or even queue one up like a missed SMS but they would rather monetize your high frequency all-time location data that they pretend to not care about.

Hopping out of "airplane mode" can be made arb fast.

So, short answer, customer demand. I bet eventually they will get too creepy with the data and demand will happen.