The reason is simple – the coach knows that repetition is the mother of skill and a tennis beginner will have to make many repetitions before he'll be able to master the stroke. And one more thing – the stroke is not only the arm movement, but consists of the movement to the ball, stopping, balancing and hitting the ball. It's a complex action which takes time to become our second nature.
You and your group of friends just started to play tennis just to have a little fun with it. You do not plan to compete or anything. Your main focus is to have fun and have a good time with your friends. You do not care about whether you have the proper technique or the form, you just want the ball to keep in play and have some long and exciting rallies with your group of friends.