Unlike any other genre, horror touches upon the Cosmic. The totality of the Cosmos is unseen and so are the best monsters of horror. Until it's too late.
Horror deals with creation (Frankenstein) and resurrection (Dracula). It deals with the end of the world (Call of Cthulhu).
I know of no other genre that digs so deep into (the worst of) human nature or so far into the hidden truths of the Universe... whether we want to see those truths or not.