The problem is that it's a network. The hard part of networking isn't the technology - it's the other people. And short of a government regulation or a monopoly, they'll never all agree.

STIR/SHAKEN has an exception for calls passing through TDM trunks, which don't support it. This created a small industry for passing spam calls through TDM trunks. Such calls can trivially have their caller ID spoofed.

The only limitation is cold hard cash. The phone companies make big bucks from telemarketers by providing an endless list of new numbers and multiple lines. The whole racket wouldn't work without the companies' greenlight.

Historically, in a world of analogue phone lines, spoofed phone numbers were a legitimate feature: if my business has more than one phone line, each of those lines is going to have its own distinct number; but I'm going to want outgoing calls from any of those lines to appear to originate from the same number (the one that I put on my promotional material, invoices, etc.).

Much the same as e-mail, identity within the phone system was, historically, entirely built on trust (because only Ma Bell could make configuration changes, and they could verify that what their customer was asking them to do was legit). Those assumptions were carried through deregulation, and are very deeply entrenched in the technical design of the network. Changing them will hopefully become easier over the next few years as analogue service is turned off entirely (e.g., BT/OR in the UK are set to turn off all analogue phone service by the end of January 2027).

It would work until you need to route an international call.

And there's a lot of telemarketing and scams happening across country lines.

I mean, they should still do it. It's just the first problem they'd face.