In Necropolis, the strength really lies in magic – specifically, raising the dead. A 3rd-level magic tower allows you to easily produce or find undead units in an external building. This is a significant advantage on maps where you don't have to play for months on end. Place some skeletons, and who can stand against you? Moreover, there's another nuance: in Impossible difficulty, skeletons are almost always deployed in the first week. No other faction has such an advantage. Therefore, Necromancers often don't even bother collecting skeletons, except in cases where there are many peasants on the map – but this is rare.