@Chook Ouija boards are successful due to something called the Idiomotor Phenomenon/Reflex. It is derived from the terms "ideo" (idea, or mental representation) and "motor" (muscular action). The phrase is most commonly used in reference to the process whereby a thought or mental image brings about a seemingly "reflexive" or automatic muscular reaction, often of minuscule degree, and potentially outside of the awareness of the subject.
Think of it like this: the glass on the board only moves when it has contact with hands. If no one were to touch it, would it still glide on its own?
If you want to give the idioemotor effect a test for yourself (without invoking the spirits of the underworld), go grab a hand-held pendulum to hover over a sheet of paper that has keywords written on it such as YES, NO and MAYBE. In response to any questions asked, the small movements in the hand will cause the pendulum to move towards key words on the paper.
Edit: Just read what @Island Man wrote above. Long story short: what he said.