They are done so with the intention of still encouraging you to interact with other players, and to group up with other players. Having the NPCs also be flawed still puts some responsibility on the player to ensure they are playing correctly, versus playing "Follow the NPC" type groove to resolve the mechanic.

If NPCs executed mechanics perfectly and did AoE combinations for standard pulls, then there would actually be very little to encourage players to group up with other people, as in many cases the NPC would just then be better than your average player, which is a backwards way of putting it.