That's because you've seen it a thousand times in every other scary movie.
However, The Cabin in the Woods played off of these normal cliches in a clever way. I won't explain in detail as to not give it away, but this movie is about ritual sacrifice. It's sort of The Hunger Games with a ritual sacrifice twist combined with every scary movie... If that makes sense.
The plot and writing (Joss Whedon and Drew Goddard, who worked together on Buffy the Vampire Slayer) are what really make this movie worth watching. They play upon the stereotypes of scary movies, referencing many things that have been done over and over again. For example, there are spirits, zombies, killer mermen, giant snakes, flying beasts, and many other horrible creatures. All these typical monsters come out all at once in an exciting, gruesome climax.
To be able to play off the irony of the stereotypical characters, they need to be very well cast. Chris Hemsworth plays the role of the alpha male, thick-headed jock well. Fran Kranz is superb as the "fool" pothead, who's theories turn out to be true this time. Jesse Williams, a bit boring as the nerdy nice guy, but it is always harder to play the straight character. Anna Hutchison has the easy character ("easy" in both senses), as all she really does is look good and scream. And Kristen Connolly - the "virgin" - also seems a bit boring at times, but has a little more intellect and character depth.
All in all, I really enjoyed this film. It's entertaining, as most horror movies are, but also has more substance.