Most obvious reason is Resource Management. In this case, Coding resources. the more complex the AI, the more server coding resources it takes to run said AI snippet, the more processing power the overall game requires, and the less available resources there are for other parts of the game (or the higher PC requirements and Network load the game will require, or both)
As general Wildlife is considered more incidental AI in the game, they are generally treated just like rank and file town NPCs that aren't used as quest nodes. They are given a limited area of influence and range of actions they can utilize.
More important NPCs, and this includes more important Hostile Mobs will hopefully have a more in-depth AI system with a a more robust area of influence and/or set of actions they can utilize. This is important as these are the NPCs in the game that the entire story revolves around. A major quest giver in a game obviously need to have more optional reactions in order to hand out the major quest chains players might pursue through them. The more quests they manage or are a node of, the deeper their coding. Same is true when you get to Boss-type monsters, larger, harder to kill monster challenges will often be only limited by the size of their lair as to how far they can pursue you, and their responses will determine all the various attack sequences they may utilize, and a more robust Boss makes for a more engaging game, overall.
As to my source for this information, I have both been a game designer in the past, and taught game design theory and I utilize my own experiences, and those of my students and contemporaries.
