Why do so many caster mobs in this game use melee?

This is something that always bugged me ever since I started. Yes, some cast mobs only cast spells unless they are silenced, but still there are a lot of mages/warlocks/priests/whatever that will run up with the utter determination to slap you to death with their staffs. A lot of mobs who do actually cast spells only ever cast one spell.
Would it not be nice if cast mobs had actual variations in what they did? A frost mage mob could have 3-5 spells they can cast instead of just frostbolt frostbolt frostbolt frostbolt. Maybe give some melee mobs weaker player abilities. In my opinion, I think that is personally a big reason why a lot of the games world is so drab and boring. Yea, an ogre and a human are different, but if they both hit you the same exact way for the same amount, why does it matter? They might as well both be humans.

I didn't find the right solution from the Internet.

References:
http://www.mmo-champion.com/threads/1916...-game-use-melee

Brand Advertising Video Example