User blog comment:The Awesome X!/Why don't the monsters use mortal weapons?/@comment-3146930-20131112231607

This is somewhat answered in The House of Hades. Because monsters, Titans, and gods are immortal, they have no concept of the future and are not meant to change. The only reason Bob did was because his memory was wiped and he now has friends. So unless an extreme force manages to change the outlook or view of a monster, Titan, or god, then they will never divert from their original forms or mentalities.