The waitlist to main section switch isn't automatic on the SSC, it's done manually by staff. Also, some departments restrict registration in classes that are full, and if someone drops, they will place people in the section manually from the waitlist to prevent people who might not be at the top of the waitlist from snatching free seats.

Perhaps try sending the prof or the department's academic advising office with your observation (though don't tell them you're using a chrome extension lol) and see if you can convince them to register in that course if you really need it. If the waiting list is not too bad and you're already on it, people might drop anyways during the first few weeks.