Heroes 6 - Discussion

Mages can also be integrated into the wastelands + Necropolis; they seem to be fighting on the same land.

The native land of mages is the desert, and nothing else. Necromancers separated from the mages, not the other way around. Simply put, the Necromancers created their own state and their own type of terrain, while the mages have always lived in the desert. If the fifth race were mages, it would be very strange, especially since the Dungeon would be unclaimed.