No healthy-ish food one could buy at MDW? (I'm not sure if that's where the OP is originating, but that's where the OP's "long" flight takes off from, if it's Southwest flying from Chicago.)

Bringing food from home may not be practical for all, especially if they're connecting to the longer international flight, and already have their carryons filled to the brim. Many people would prefer their food not to be hours old, which it would be if they brought from home onto a connecting flight to then transfer to the international flight and eat it there.

(The OP only mentioned the flight they were asking about; I can't tell from their post or profile if MDW is close to home or not. And, anyway, it's most likely a round trip, and in that case home is nowhere near the other end of the flight!)