Why are the workouts so short?

Intensity is the key. There are some workouts that take 5 minutes to complete, and some that take 25 minutes. It’s often a certain amount of work to be completed in the least amount of time, or a certain amount of work to be completed as many times a possible in a given amount of time. Sometimes the workouts have no time constraints like a maximum load WOD. In all of the above instances, intensity is the key- as much as you can, or as fast as you can, or as many times as you can. All favourable adaptations of exercise occur through increasing intensity. Long workouts are, by definition, less intense than shorter workouts. Your body cannot operate at a high intensity for very long.