That's because the combat is one dimensional. No matter what the trash mobs might be capable of, they have to be beatable by a group of people with one tank, one healer, and two damage dealers. Abilities to redirect aggro or crowd control monsters are not consistent to any particular role, so they cannot be strictly enforced as requirements for beating a pack of trash mobs (aka, no "CC or die" trash pulls). On the other hand, they can't have enemies use crowd control abilities on anything except the DPS because of a reliance on long cooldowns and melee stuns as the only effective counters. That means countering enemy spells cannot be strictly enforced as a win condition against trash mobs either. Finally, they can't create ambushes of any real worth because enemies do not respond to physical obstacles that would allow for choke points to be effective.