I suspect that sheaths are typically omitted because once the modellers tell the producers the amount of money and work which would be needed to get to the sheaths to properly clip and look good for any sort of weapon, and on any sort of clothing, and on any sort of creature who carries a weapon needing a sheathe,
Yeah, I would rather have unique looking weapons, then generic looking weapons with sheaths. One could do a little scaling to model and try to attach weapons in more believable way though - like swords and daggers hanging from hip and such.
Levitating weapons on back though have an advantage of being clearly visible to the player.