There are a few problems with what you said:
1. People can't purchase tickets directly to DAL
2. Landing/Take-off wastes a whole bunch of fuel, which is not something that SWA takes lightly especially with today's prices
3. Turnaround times average 30 minutes. Usually luggage/freight (yes SWA carries freight) is what holds things up, not people.
4. Because of #1, SWA can't provide cheap prices for flights to/from DAL.