Horror is a protean genre, spawning numerous subgenres and hybrid variants, including gothic horror, supernatural horror, monster movies, psychological horror, splatter films, slasher films, body horror, comedy horror, serial killer films, and postmodern horror.
The main reason it's not terrifying is that most of the horror film happens in broad daylight, not in the characteristic darkness of other movies in the genre. Another reason it's less frightening is that nothing overtly supernatural happens in the movie.