Why don't monsters use all the cool weapons in the dungeon?

You know the drill. You have a dungeon filled with dangerous monsters and undead guardians, and the dungeon has rare and magical weapons inside. Why aren't the monsters inside using them? Why are they using generic rusty spears and stuff instead of the +2 Magic Sword in the ancient vault? Here's some reasons.

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8 Reasons
[1] Monsters rely on their innate strength, view actually good weapons as "pansy ass human shit"
[2] The weapons in the dungeon are like collector's items. Don't want them to get dirty.
[3] Too dumb and/or superstitious. Trust what they've made, anything else could be cursed.
[4] Religious reasons. Everything in this dungeon is tribute to their dark gods, can't use them.
[5] Monsters are either created/enchanted to serve the dungeon's master. They are following orders.
[6] Some monsters (like skeletons) are reliant on magical fields to stay together. Powerful magical items, beneficial or not, will essentially overload them and make them fall apart.
[7] They're actually in the dungeon to find those weapons. If you take too long to loot the place, they may eventually unearth them and start attacking human settlements.
[8] Magical weapons are reserved for their ruling class and those in positions of honor. Regular units don't use them; but their captains might!