[afro88]

When your eyes look at an object in 3D space, they not only swivel so both are looking at it at the same time (convergence), but your eye muscles manipulate the lenses so the object is clear and not blurry (focus). Think of how a camera focusses it's lens to bring an object into clear sharp view.

So in the case of the Oculus, the lenses in your eyes remain focussed on the images on the screens right in front of you, rather than changing depending on how far away the object is in the virtual scene. So there's a disparity there that causes your brain to go wtf and stops it from fully accepting what it's seeing as real.