Why is there a minimum for each app?

The short answer: to help you create a quality app.
Apps submitted to apple are reviewed for quality control, we only accept apps who have the minimum number of content screens, pictures, texts, etc. but even then we prefer to accept the highest quality of those we receive. Even from that selection, we cannot guarantee that Apple will accept the ones we submit. To ensure the best chances, when creating your app, be sure to surprise the minimum requirement of screens/pictures/texts mentioned in the builder.