As animals, humans have the instinct to be afraid of anything that might kill us, and that's where we find the focus of most of our stories. Whether movie or book, the protagonist is always faced with a kind of danger to their survival. That is what strikes a cord in us, makes us empathize with their struggles and vicariously get scared. It's especially frightening when the movie ends like ones such as the ring did, introducing an video/image that kills whoever views it, then showing that video/image to make you fear for your own life. It may be idiotic to believe a ten second clip of people in spooky masks could kill you, but anything that threatens survival is bound to send a chill down your spine.
Now, for some time now, the genre seems to be more focused on simple, almost frustrating jump-scares (where the viewer is scared by a sudden loud noise and image). It's cheap, but there's science behind these too. It always comes back to us not wanting to be eaten. Many animals stalk prey, and in a split second their air bound and coming at you. We evolved to recognize this threat and instantly run, or in this case, flinch severely. It sucks, and to all hardcore horror fans the best movies go without them, but it gets the point across that this thing is a threat and could kill you, even though it's just CGI.
There are a lot of bad horror movies. It's a fact. I'm not the only one who thinks this, apparently there are many, many people, and some of these people just happen to be movie producers. Cabin in the Woods, Shaun of the Dead, Zombie land. All labelled as comedies, and all with one goal in mind. To trash the horror genre's bad apples. They find all the cliche's (such as jump scares) and stuff them full of irony, putting reason behind it or emphasizing it to the point of comedy (hence the title; horror comedy).