They might have an accomplice within carriers' or dealers' employees. Imagine how many shops are there across the country and how many employees have access to reissuing SIM cards. As an excuse such an employee can always say that they didn't notice that the documents were fake.

at least in germany ordering new sim cards to new addresses your provider never heard of before was a thing some years ago. I think porting a number is a lot easier if you know the detailed process though.