Necromancers liked areas controlled by Beastmen the most. In those areas, they could secretly dig up many skeletons from the Beastmen's cemeteries. These skeletons were good materials for making Undead creatures.
Unfortunately, these skeletons' souls had already returned to the embrace of the earth. Even the most powerful Necromancers could only turn the dead Beastmen's corpses into the most ordinary Skeleton Warriors.
Even the skeletons of Beastman powerhouses could only turn into Skeleton Warriors. They would just be a bit more powerful.
And these Skeleton Warriors were the most common cannon fodder used for the standard Undead of overwhelming with numbers.
Moreover, Necromancers wouldn't dare to tamper with the corpses of the Beastmen who turned into Ancestral Souls. Turning one into one of the Undead would be the same as creating a tracking device giving out their location. They could only wait for the Beastmen to chase them down then.