The same reason why to-hit tables were moved from the DMG to the PHB -- because it's information players need (and preventing players from getting it is futile). Player characters carry around magic items and monsters do not.
Monsters not carrying around items is a 4th edition thing and nothing more. Player's don't get magic items unless the DM says they do or provides a means to get them. Not allowing monsters to have magic items was stupid on 4th editions part. A perfectly good magical sword just sitting there in a chest or a hoard and not used by the creature made no sense what so ever. I don't want the bad designs of 4th edition to fall into the next edition.