Don’t forget to pay your exorcist bills, or you’ll be repossessed!
She was 13 or 14 at the time of The Exorcist, so… 
But I think even the most determined pervert would be put off by her vomiting pea soup… oh… Rule #34… nevermind.
My guess is, because it’s not nearly as much fun for the demons as it’s cracked up to be.
Yes. he schiezophernia or other mental illness which doesn’t respond well to medication or other interventions.
Ooooh, looks like somebody had a run-in with an exorcist! 
In short: occam’s razor. Okay you added demons to the brain. So I add elves to your demons. I say they control the demons so its really an elven possession. Now you add fairies to my elves. Okay, so I add orgres to those faeries. At a certain point you need to check your assumptions if you want to be rational.
For instance, if you get a cold, we know its a viral infection. We dont say your humours are also out of whack or that you were cursed by a gypsy. The one explanation works just fine.
Hey, thanks for not addressing anything. Good job.
I think kanicbird actually has a good point. It’s probably that tons of people are possessed, but we don’t acknowledge it. People that are depressed or have anger issues or whatnot, that’s a demon. We just don’t recognize it as such.
Watch out for that demon of handwriting analysis… it’s a doozy.
Ladies and gents, your demon.
That makes me think that we have advanced from being in fear from the prince of darkness, to being in fear of the prince of insufficient light. 
Really, when demons can be defeated with a pill or other treatments, one should question if they are demons at all.
But the harm of denying that progress comes more often from old time religion:
http://psychcentral.com/news/2008/10/16/clergy-often-downplay-mental-illness/3147.html
Because the leprechauns are protecting us.
Cookies!
Probably because its not falsifiable. Theres a poster I have known on other boards who claims to be the habitation of demons and the mistress of satan. For real. I even met her, she seemed normal enough.
Hmm, perhaps it is because with all the emphasis on self expression and self esteem we no longer have room for the demons, as we are so full of ourselves? As I recall the demons need both a flaw to work on and a lack of vigilance or an open welcoming. Could it be the emphasis of secular humanist culture on narcissistic behavior has rendered modern man somewhat immune from demon possession.
Is it “Moriah- Conquering Wind?” I’ve run into her once on a Christian Universalism board. I love it when she types like Gollum speaks!
That’s just demon propaganda, designed to lull you into a false sense of security.
Yes, by all means, let’s leave reality out of the equation when addressing these issues. Let’s get back to talking about invisible demons that cause anger and depression. Reality? … harrumph. Demons (I’m assuming with little celestial lug wrenches going to town on ones neurons) – that’s where the rational points will be found.
If I recall aright, the senior priest in The Exorcist averred that the point of Regan’s possession wasn’t her damnation–she was obviously blameless–but to cause the persons around her–her mother, obviously, and maybe even the priests sent to help her–to commit the sin of despair. Regan was just the instrument; the demon could also have caused her to die horribly, but apparently found this more amusing.
Then how do you explain Nancy Grace? 
More seriously, I am choosing to consider this question in terms of the mythology. It’s not fundamentally different than my Isildur thread in CS.
Another reason for the paucity of demon possessions is that the number of demons, though vast, is not infinite; likewise their resources, though staggering, are not endless. I believe the myth of the War in Heaven states that a third of the Angelic Host got tossed out by Michael & Co. Saying “one-third” implies a finite number of the original Host and thus of the Fallen; and the fact that a war was fought that took a measurable amount of time implies that neither side was of illimitable might or knowledge. If we imagine that the Fallen Host numbers, say, 100,000,000, opposed by a Heavenly Host of 300,000,000, it’s easy to see that they might have to pick their battles (even though the single mightiest angel is on the side fo the Fallen). They’d use different tactics in differnt contexts. For a high-value mortal target – say Martin Luther King, or George W. Bush – they might assign multiple demons to tempt, or perhaps to simply try to kill; other demons may be assigned multiple lower-value targets. Also, since possession does not affect the redemption/damnation status of the possessed person, and presumably prevents the demon from doing anything else, Fallen will only be assigned such a duty if it can be used to help damn a valuable target.
Why would Demons make it obvious that you’re being messed with? They’d have plenty of ways to fuck you up and ruin your life without you ever knowing what they’re doing.
Assuming that they exist, which is the premise of the OP.
Actually, I answered the title question conclusively. There are no demons, devils, gods, spirits, fairies, pixies, sprites, or other supernatural entities. There never have been.