More specifically

The overrid gear of the optical drive rotates when you push it. When it rotates, it is almost impossible to avoid the teeth from hitting the pencil lead. Even if the lead doesn't break at the time it is pushing the gear, it will definitely break when the gear teeth hit it unless it is really thick and tough.
In fact, I don't see any possiblity for most people to have "thick and tough" pencil refill at home.