Edit: Also the political situation at the time was that if Clipper was adopted, all other encryption technology would be outlawed. It was a very scary time.
The key word here being "should", and they are clearly not filtering at the network level. I can tell, I still get spoofed robocalls... The clipper chip provided end users the ability to cryptographically prove callers were who they claimed to be independent of the network operator, which is a key need of a crypto system.
Robocalling with spoofed ANI has been a problem long enough that I think we can safely say the network operator is 100% complicit with this activity now. The phone company could track down kids war-dialing blocks of numbers in the 1980's in order to make sure they were not telemarketers not paying higher telemarketing fees. Do people really think the phone company is not getting a cut of these robospoofers?
There is actually encryption technology[1] available that would solve this or at least make it traceable and blockable. But here we are.
[1] https://en.wikipedia.org/wiki/STIR/SHAKEN
Edit: Also the political situation at the time was that if Clipper was adopted, all other encryption technology would be outlawed. It was a very scary time.