It might have something to do with the bookings of flights from Love Field, their home base. There is this stupid law that prevents from flying to any state that does not border with Texas, with the exception of a couple places. This law was put in place to make sure DFW airport would be the large airport for the area.
It could also be that SW likes to control all aspect of booking. I don't recall seeing them on any travel website either. Just my speculation
darren