Boarding time is a big factor in the cost structure of airlines. If they have to check ID of everyone who boards, it’s a lot slower. And in fact European airlines have a much slower turnaround time than Southwest. There’s already an ID check before you get airside, so another check during boarding is not needed for security. If all it accomplishes is revealing this edge case, which can be deterred by detection and enforcement after the fact, is it really worth making everyone wait and increasing costs?