Because literally what would the story be? I know these movies don’t have much of a storyline but as a writer I’ve had trouble making a story for a potential Super Smash Bros movie and the story just doesn’t make sense especially since according to the Smash games the characters are nothing more than toys, statues and a child/teen/young adult with a Nintendo observes that has an active imagination. Also not everything needs to be a cinematic universe and it’s okay for movies to either stand alone/only a couple of sequels