It’s all because of a retained reflex called the STNR - symmetrical tonic neck reflex. It’s a reflex that splits the body in half from top and bottom, and it helps to allow babies to learn to crawl forward. When it doesn’t integrate, or mature, that leaves the person’s body movements at the mercy of what their head, neck, and eyes are doing. It’s extremely exhausting! It can be 10 times harder for people with this reflex retained to copy from the board, sit properly in a chair, ride a bike, etc.