It is actually because you can't do anything with the source phone number being tied to an identity if the scammers are freely able to spoof the number that the end networks and users see. Even if every network in the world adopted this it wouldn't matter if caller id isn't fixed so that you can actually see the source number to go ask who it is!

They can also use VoIP. So just banning calls from abroad is not enough, you need to better regulate VoIP services.