Oh, that's fairly simple -
Animate Dead is an (Neutral/Evil)Cleric 3, Death Domain 3 (class level 5), Sorceror/Wizard 4 spell (Class level 7 for Wizards, 8 for Sorcerors), useful for creating cheap gaurds for an area. It's possible that the bandits have a few keeping watch so they don't need to worry about it - after all, zombies and skeletons don't require pay, food, or time off (they aren't unionized, bandits are).