The clerk has to use some kind of online system to connect the new sim to the customers phone number. The system would obviously require the clerk to authenticate himself and could require him to enter the passport number or other document ID he checked to verify the customers identity.
If later it turns out this was a sim swapping attack you can verify if the clerk entered a valid document ID. He can’t do that without having been presented a proper document, so you can tell if he checked.
Its just convenience over security. Lot of things can be done but then the extra burden that companies have to go through. Think about that people don't use app based authentication because it's inconvenient even though it matters to them. How can you expect carriers to do it
I wasn't sure how would you solve the problem of verifying the ID card without showing the previously recorded number to the clerk. But simply requiring to every time just punch in the ID (and maybe scan the whole card to check the photo later) could work - if the system only returns a big OK or BAD signal.
Currently here, in Hungary, the clerks just photocopy the IDs though. And there was a big scandal a few years ago (in connection to the ISIL/ISIS attacks in EU) about some groups obtaining hundreds of thousands of SIMs for just a few names.
