You're right, it is an easy technical fix.

Mainland China lets people opt out of phone calls that come from outside of the Mainland...it's a feature one can turn off on an on their mobile plan.

Calls from outside the Mainland always cause a warning to pop up on the receiving user's phone that says something like "this call is coming from outside of the mainland, be careful of being scammed".

I can imagine there are many reasons the US doesn't fix this..one of which probably that much of US customer service is outsourced to people outside the US!