The reason why is called the "gorilla-arm problem" -- human beings are designed in such a way that holding your hand up in the air for long periods, making many fine-grained motions, isn't particularly pleasant. Tack a sheet of paper to the wall at eye level and try writing on it while seated to see what I mean. Light-pen-based systems manufacturers learned this the hard way.

Surface-mounting the touchpad-display in your desk solves this problem, but that requires specialized furniture.