After awhile, you can truly see how fake the effects are. The peeling back of skins, the blood oozing out, the cuts--you can really dissect it. Very fake looking. Especially when you see the same actors over and over again, your mind is like, "ok, everyone's all right even though they've been slashed and tortured". That just reinforces the fakeness of the whole thing.
That's why when I watch these kind of movies, I've grown pretty numb to all the gore because my mind is telling me that it's impossible for something this sick to occur in real life--unless if you are just that sick (in which case, that person is mentally disturbed).
