Well, let's see... A bit of internet searching shows SWA has about 60,000 total employees. Roughly 6,000 of those are pilots. So probably about 10,000 cabin crew (3 or 4 flight attendants, 2 pilots per flight). Most of those 15,000+ people need to be in exactly the expected place at the expected time, or things will get snarled up. If any one of those 5 or 6 people is a no-show then the plane can't legally fly passengers. There's limited fungibility; one of the pilots has to be a Captain. You don't want two Captains flying, if for no other reason than some other flight will then be short a Captain. One of the cabin crew has to be a Purser (supervisor). It's really preferable to have at least one of the pilots flying to the airports they usually frequent. Off-shift crew still need to be in the proper place at the proper time, or the plane has to stop mid-cycle. Then there's the ground and gate crews.

And you can't just say "go to your usual starting airport". Flights are 24/7. Days of the week matter. Holidays matter. Even your local burger joint doesn't have cast-in-stone schedules. Much less a huge interconnected network that's trying to reboot. Even if 80% of the crew could go to a "usual airport", how would you know if you were in the 20%? Oh... everybody has to phone in, or the system has to send a huge number of notifications out. System crash. (Remember, normally the system depends on scheduling in advance, and only a few last-minute changes need to be handled.)

And how do these what, 20k+ people get where they should be? If they aren't already at that airport, then normally by hopping on a SWA flight. Which aren't happening. So now what? Book flights on other airlines? Who's going to be doing the booking? How does it get paid? How many employees have a company credit card (any?) Use the employee's cards? How many are maxed out for Christmas?

So let's suppose you're scheduled everyone and everybody knows where there's supposed to be and when. What happens when 10% call back in (without crashing the system!) and say they can't get there on time. (Remember, many SWA customers are stuck and are having a hard time arraning alternate travel. Crew without access to SWA flights would have the same trouble.) You're going to either apply massive changes to the existing schedule, or start all over and reschedule everyone. Which is exactly the problem they're currently having. They can't handle massive corrections. What's worse, a crew member may think they're good, and then their travel gets delayed somehow.

How sensitive is the scheduling? If 10% of the crew are no-show or delayed then about 9% of the planes are affected; every 8 hours 90 planes will have the following 8 hours of flights cancelled or delayed (throwing further monkey wrenches in the schedule). About 40 of those planes will be missing a pilot, which means they can't even be deadheaded to where they're supposed to be next.

So how to reboot? They've had to cancel two-thirds of their flights, so apparently they're able to keep 1/3 of them going. Keep those flying so they can shuttle crew around. You're initially only scheduling 1/3 of the crew, so the poor overloaded system can handle it. 2/3 of the flights are just outright cancelled, days in advance, so the customer support load is reduced. ("I'm sorry, your flight has been cancelled. We can't reschedule you until x days from now." vs. much back-and-forth trying to find something with an overloaded system.) Slowly add additional crew and flights so the number of phone-ins is kept manageable.