Why are roll ups and roll downs in Pilates, not a good “core” exercise?

It IS a good core exercise, but I recommend extending actively through the legs and heels as one comes up. I also recommend, as Marie Powers mentioned, to inhale as the arms come up toward the ceiling, and exhale as the arms drop towards the torso. It also helps to press your feet against a wall if you are having trouble coming up. Half roll-ups are the way to go if you find yourself “jerking” to come up all of the way. Scapula must be snug to the back and not lifting toward the ears. Pressing through the crown of the head (with chin tucked a bit) when one comes up, helps to NOT work the neck or shoulders too much. R Laney-Meers, Jackson, MS