[InclinedPlane]

Static photorealistic scenes are still a pretty hard problem. If it was just "ray tracing and you're done" it'd be a different story. You have complex lighting interactions (best modelled using the radiosity technique) and you have complex sub-surface light interactions which are a prominent compnent to making ordinary human skin appear real. These aren't even completely solved problems in the state of the art today.

Now, all of that is just square zero. A mere preface to the real problems: realistic animation and realistic interactivity. You can make a pretty decent photorealistic world with current technology but it would have to be an untouchable, unmoving, unrealistic frozen world. Adding in animations is a problem at the same computational intensity as the rendering problem. And making the world capable of being interacted with a level of fidelity approaching the quality of the rendering and animation is a problem so lacking in proposed solutions that it doesn't even have a sound theoretical underpinning yet.