But lots of airlines have come and gone or merged over the years and these systems would have already had to deal with new airlines codes?
In prior roles, I've worked at 2 of the largest US airlines that have gone through mergers. There are lots of hard-coded letters in decades-old code that help identify what's mainline, regional, and OA (other airline). We're talking everything from reservations (booking) to revenue recognition (flight departure).
It would be no simple feat to upgrade the tech stack. Hell, some of the mergers are still lingering within the systems because airlines wanted to complete it quickly for the passengers. The backend, however, has band-aids all over the place.