For the same reason that every movie ends up ripped on piracy sites, but you still can't watch Netflix in 4k on Firefox on Linux.
DRM doesn't work because it only takes one person to bypass it to make a copy, and caller ID verification doesn't work because it only takes one janky provider that doesn't implement SHAKEN/STIR correctly and yet is worth too much money to totally block.
FWIW I can still generate calls with arbitrary caller ID from a handful of my (legacy) ITSP providers, but if I get a new account today with any of them, they will require me to either verify each caller ID by receiving an inbound call or provide a "valid business justification" for why I can't do that. They are working on tightening up the pathways to generating fake caller IDs but in the telephony world, nothing moves fast and uptime is more important than anything, except maybe revenue, of which spam calls account for a ton.
Well, if legislators are trying to fix it, I suppose I feel better about the user hostility. Good luck to the legislators, and thank god we have people like them!
