Because they have no direct relation to the actual plot, some even happened in the middle of actual storylines, when it was impossible for them to happen. Movies are considered non-canon because usually Kishimoto isn't the creator of its content, just like the anime-only scenes. That is why they aren't considered part of the actual storyline. However, The Last: Naruto the Movie takes place in between chapter 699 and 700, and the story and characters were all developed by Kishimoto, making it canon.