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

…to activate the deep core muscles( local stabilizers) you have to place your spine in a neutral position! Then you have to challenge the spine moving arms and legs to change its position! As an example siting on a dome of a BOSU Keeping Spine Neutral and legs in inverted V position( V-Sit)..then start toe taping only. Thats CORE Stability!