You can cancel your Southwest flight up to 10 minutes before scheduled departure, if you do that SW really can't sell your seat to someone else.
Airlines need revenue certainty. Non refundable fares give airlines revenue certainty, in return you get the lowest possible price. You don't lose anything unless you don;t fly again.
You can get refundability, and the airline loses revenue certainty. Just buy a much higher priced refundable fare. The airline is willing to give up revenue certainty in return for the higher fare.
