Because sometimes that isnt necessary. Sometimes less is more. When writing,especially fiction, you do so to tell a story. Sometimes that story spans chapters, sometimes only a few hundred words. Quaninrty isn't as important as the quality. Some ideas become streched too thin when made into longer stories, while others require the longer length. So the reason why varies. Perhaps a writer doesn't wish to have to come up with more details or plot. Or perhaps the writer has told the story in its entirety, with nothing left to tell