Once, there was a Golden Age, where beings with powers beyond imagining walked the Earth. Now, however, all of the gods, really strong mages, Precursors and whatnot are gone, or shadows of what they once were. Fortunately, this means the great demons and monsters are also weaker than they once were, but unfortunately, modern society is far less capable of dealing with them. For more information on this trope, consult The Encyclopedia of Fantasy's entry on "Thinning".