Many horror film have misunderstood what generates the greatest amount of fear: the unknown. The first «Alien» movie was frightening, because we had no idea what the creature looked like or where it was. Towards the end of the movie, we finally got the reward of seeing the horrific creature in its whole grandeur.
Once a horror movie starts to flaunter its monsters excessively, then the «unknown» part becomes… well… known. The result is that only shock value is left to fright us. The movie has to rely on «what is behind you right now?» and «What is around the corner?» to give you value for your money.