User blog comment:The Awesome X!/Why don't the monsters use mortal weapons?/@comment-25060215-20140623160921

In Lost Hero Annabeth asks Piper if she wants a gun but she refuses because it's "to flashy", also in Titan's Curse Mr. Chase uses his plane and shoots them with celestial bronze bullets. Also in Titan's Curse, Atlas has the plane that that Artemis turns into crows (the plane was some person's originally). I think it is because they don't see reason for them and being so old they probably don't grasp the knowledge of how to use it.