When it comes to exercise, something that is important is progress. We must always be striving to get better in some way, whether that be to lift more weight, move more efficiently or do the same amount of work in a shorter time. Our bodies don't get much out of random workouts. It is progression that drives changes in our body.
Because of this, we keep the workouts the same for a few weeks to give us enough time to progress with them. Each time we do a workout we try and improve on the previous one, so although the movements might be the same, it is our intensity or overall workload that changes.