It's the same reason that 'scary' movies aren't scary any more. They follow a set pattern and concentrate or the gore/graphics factor rather than a good scary. The original Silent Hill and Resident Evil games had great story lines, backed up by good graphics, not great graphics backed up by a vague reason for why you are constantly blasting something's head off.
Quarkzquarkz and SchizoFrog hit the main two points. First, we've had a period where there have been A LOT of horror games and movies. The same thing happened back when the original Halloween movies came out. Everyone started making them and then the general public became desensitized and turned away from horror for a bit.
Second, because of the prospect that the genre would tail off, the makers have tried to find a way to keep the train rolling. That has resulted in more focus on the thrill aspects (gore) rather than real fear.
I would also add that the reality of some motifs have been demystified. Zombies were never the most terrifying of the classic monsters, so they've been re-imagined as not "supernatural" in the classic sense, but more like mutations caused by humans. Nevertheless they have been done to death of late, further making it seem like every possible story has been done. Then you have the "glinting" of vampires thanks to the likes of Twilight.
It's a matter of perspective. If you've watched tons and tons of horror movies, played horror games, etc., you have a much deeper knowledge and experience with the genre than most people. It's obvious you'll start to feel that the genre's hit a wall or stopped innovating. It's the same way that as people get older they start complaining about how new pop music is terrible and start citing pop songs from their time.
Theyre not really horror games they're action/horror games. Horror games are based around the spooky factor of the game and its story while action/horror games are based around the fun of killing the spooky thing.
We are not that desentisized. It is just that we are given too much power in most horror games. Ample supply of ammo, powerful weapons and almost Chuck Norris level of hand to hand combat. Most horror games today are really action games with horror elements. Gives us more RE1 style of horror. Hell, give us more Amensia style of games. If they want action horror games then they only need to look at Dead Space 3: Awakened. That DLC should have been what the main game was.
I think it will get progressively harder to make scarier games because nothing is "new" anymore. 20 years ago, the original Doom was scary when you played it late at night -- today, it's just a bunch of pixels.