I'd assume a nontrivial number of customers are calling because automated methods to reschedule aren't giving them the options that they want, and they assume (probably incorrectly) that they will do better when talking to a human.

I don't know with Southwest, but I know more than one airline has a system that sends automatic "choose one of these options" emails when flights are cancelled. When all of your customers need new flights, though, the issue isn't how the rescheduling is done, it's the fact that almost nobody is going to be able to choose an option they like.