The telephone network is international. The US is limited in its ability to enforce anti-spam laws against operations running out of other countries. (They also seem unwilling or unable to enforce these laws domestically, but that's a completely separate issue.)
In this case I think it's a case of being unable to enforce the laws, because the technology for verifying callers is broken, making it easy to spoof phone numbers--even the US government doesn't know who is calling you illegally. SHAKEN/STIR is intended to close the security hole and make caller ID accurate again.
TL;DR is that it's "unwilling by way of almost unable".
The cooperation needed between telcos, who already have overly complex and archaic internal systems to manage themselves, would be almost impossible to coordinate.
