Here's why:
* Short stories are primarily a form of narrative. They aim to tell a story, and often explore themes, characters, and situations that are imagined.
* Non-fiction short stories do exist and are often called "true stories" or "personal essays." These stories are based on real events and experiences. However, they are still narratives, meaning they use literary techniques and creative writing to present the information.
* The distinction between fiction and non-fiction is based on whether the story is based on factual events or imagined events.
Therefore, when someone talks about a "short story," they are generally referring to a fictional narrative.