Because most demons in demon possession flicks are stupid. They're too busy just being a pain in mortals' rears to actually use their power for anything worthwhile. They're all like "I'm pissed off!" this, and "Untie me from this bed!" that. If I were a demon and I possessed somebody, I'd be raining fire from the sky, not spitting up pea soup while tied to a freakin' bed. Or I'd be sneaky-like, undermining the political system (or something similar) from the inside. Screw this petty temper tantrum crap.

Demons deserve respect. Few demon possession movies give them that. Their victims tend to be like "Oh, crap, now I know why I'm afraid of the dark!" moreso than "Holy mother of God; what great evil is upon us?!" To me, they fail to inspire either fear or temptation.

There are a couple of exceptions, though. Fallen was pretty good. Azazel at least walked and talked and used the bodies it possessed for something. I'm told End of Days is good, albeit an Arnold movie.

Besides all that, I like to see the possessing entity move from one host to another, and you rarely get that in a demon possession flick. Even Fallen did nothing more than a keen sound effect.