Your eyes were maybe the first thing you ever learned to use. You could focus on a rattle long before you could put it in your mouth. You learned how to use your eyes at such an early age that you don't even remember that you ever had to learn.
Moving your eyes is so ingrained in your subconscious behaviour that you rarely use your conscious mind to move them. This is useful to researchers because it offers a window into the subconscious mind, and this can be observed and recorded with great fidelity and accuracy...