Here's why:
* It's a horror movie: The primary purpose of "Paranormal Activity" is to scare the audience. The filmmakers use various techniques to achieve this, including jump scares, suspenseful scenes, and the suggestion of paranormal activity.
* There's no evidence: Despite the film's realistic presentation, there is no actual evidence of paranormal activity beyond what's presented within the fictional narrative.
* The story is fictional: The film has a plot, characters, and a narrative arc, which are all common elements of fiction.
While some people may find the film convincing, it's important to remember that it's a fictional story, not a documentary. The filmmakers use their creative license to craft a scary and entertaining experience.