Stories have macro-beats and micro-beats. Macro-beats are like a skeleton. They lend a story shape and structure. Micro-beats are everything that covers the skeleton: muscles, ligaments, blood. They are what make the skeleton interesting. As Stephen Sondheim says, "God is in the details."
Think of music. Think of the first notes of the Lord of the Rings theme. The skeleton of the measure is: D C D. (The notes could be wrong. I don't play music.) On its own, not unpleasant, but a little boring. Now add the staccato notes: D C DD C. Open GarageBand and try it. Instantly more interesting. The three notes on their own show potential, but don't have the sound of a finished thing. They are the bones. Adding the second D brings the piece to life.
So with story. Many short stories feel like summaries of longer stories because the author tried to cram a novel into ten pages. Only the bones would fit, and so the piece feels lifeless.