Reality is a lot more flexible than most people want to believe. No one wants monsters under their bed or lurking in their closet; they want to say "it's all safe here," close the doors, and know that they've seen the end of the nightmares, the last of the danger. No one wants the things in the shadows to be real. It's a pity that the things in the shadows don't agree.
Part of the challenge of protecting a world that isn't quite ready to admit than monsters might be real is keeping those monsters from forcing the issue. After all, there's no magic to keep people from noticing, no special set of rules that keeps the fight on a different plane of existence. There's just the fight, over and over, until somebody wins. And that may never happen.
Reality is a shifting target. When you make your throw, it's hard to know just what you're going to hit.