Why do short story paragraphs only have one or two sentences?

A paragraph is 4 or more sentences, so having one or 2 doesn't make it paragraph. Short stories aren't short due to the number of sentences in a paragraphs, but to the number of pages in the story. If a student turns in a story to me that is only one or two sentences in each paragraph I hand it back telling them to write real paragraphs with a topic sentence and 3 detail sentences.