I think you miss the whole point. Legacy carriers use regionals to feed the mainline system - in other words, to fill up their flights. Why would Southwest need a regional support network to feed their flights - their flights are already full. No need to pay (read = share profit) someone to put pax in seats when they can do it themselves just as easily and keep all the money.
Besides, the way Southwest see's it:
They can pay some reginal captain $50,000 a year to fly 50 pax at a time between PHX and LAX or they can pay one of their own Captains $150,000 to fly 150 pax at a time between PHX and LAX - what's the difference - so again, why not just do it themselves.