Commonplace Monsters
Sometimes a quote says it best:
“One cannot, except in immature pulp charlatan-fiction, present an account of impossible, improbable, or inconceivable phenomena as a commonplace narrative of objective acts and conventional emotions. Inconceivable events and conditions have a special handicap to over come, and this can be accomplished only through the maintenance of a careful realism in every phase of the story except that touching on the one given marvel.” — H. P. Lovecraft (Notes On Writing Weird Fiction)
I wrote about this once before, this time Lovecraft explains why, and I cannot argue with him. When your monsters are commonplace they lose that special quality which makes the story otherworldly, weird, or supernatural. In some sense we have made the supernatural natural and thus removed the mystery and appeal.
(I’d recommend reading that essay of Lovecraft’s. “Google” it. I’d like to link to it but I don’t want to mess with ambiguous copyright status.)