"but they are also designed to display a precreated set of items and effects, and optimized to handle that."

Well yes and no and you can't forget about mods. Mods turn these games into completely different games. All new models and textures and worlds are used. Sure often they do take a bit of a performance toll but they still look great. Not only that most companies who make these games sell their game engines to be used in completely different games. The new Doom 3 engine and UT2007 engines will be used in TONS of different games. So yes, they optimized, but they are flexible to add in anything these mod makers can think of.

The ultimate point is, it looks good, incredibly good. Even if the final render is done in the standard slow way, imagine being able to set up a world and walk through it. Use physics to have objects lay around and just keep on moving till you find just the exact spot you want in real time. Do this in game quality and then do a final render. Now that would be awesome! :)