The point is that reality is little more than what you experience. All that simulating a reality requires is that an individual's experiences are able to be constrained to within the controlled ream. Take a child, 'disable' all senses except sight - and install permanent VR goggles. That child, lacking outside interference, will believe in a reality that is not really real. Should it develop the cognition to wonder it will likely do as we do. Why do large bodies attract one another? We can explain the operation of this through perhaps the curvature of spacetime, but that then again one can ask then why is there spacetime? At some level the answer simply ends up being that because that is the way it is - a inherent facet of our universe.