It wasn't "all a dream"
The scene with the girl waiting for the phone to ring, slowly grinning when it finally does, with the man in the bag next to her rolling over when he hears the phone... that scene happens waaaaay in the beginning of the film, before they even go for the weekend break in the hotel and dreams start to kick in.
Are there dreams in the film? Hell yes, but not all of the horror element happens within dreams.
At the very least, she has a mutilated dude as a pet in her house ;)