Because movement isn't 1% of the brain. Movement is why we have brains. Balance and head-righting do sensor fusion between vision, the vestibular system, and proprioception. That's a whole-body problem, and I don't see how that can be a low-energy activity. Brains are expensive.

We do have an unconscious falling reflex, but that works by startling us awake! Actually doing movement planning requires a running brain.