I've seen two reasons. 1) Backward compatibility--you would have to add an additional security layer, and some old legacy switching hardware (5% of the network

I've seen two reasons. 1) Backward compatibility--you would have to add an additional security layer, and some old legacy switching hardware (5% of the network?) wouldn't support this. 2) Phone companies profit from the calls, so they have no incentive to spend the money to upgrade and stop it.