The path of least resistance
Drawing with your whole arm is, frankly, difficult. Or more accurately, it's unfamiliar and unpracticed. We've spent years writing from our wrists, and so the muscles involved are far better developed than those needed to draw with your entire arm. As such, we end up facing more resistance, especially as beginners, when we try to use it. It gets exhausting, and we get sore quickly. That muscle weakness can also make it harder to be confident with our linework.
That's why when we face the choice of how we're going to use our arm to make a mark, when we're faced with two paths to choose from, using our whole arm poses far more resistance. We naturally want to go down the easier road, to draw in the familiar fashion, from the wrist. It is for this reason that this course forces you to draw from your shoulder in most cases.
I cannot stress this enough: be patient with yourself. Do not rush, do not force yourself. Take breaks, and allow the work to take as long as it needs to. You only have one body, and to break it so early would be a tragedy.