Many people don't like it when their shifts change dramatically. Sure, you can cover the night when half the staff takes off to see Taylor Swift, but calling all those people in means they in turn need their next shift covered, and so forth, and you end up with an entirely different schedule of people who have never worked together, etc. This is all fixable adding more constraints, but even just writing it all down and prioritizing them isn't trivial. They're not Legos.