It's important to realize that what happens in post is a whole lot more than making a 3d object and placing it in the scene. We do a lot of integration work in 2D by hand to match things like color, edges, etc. There's massive amount of simulation work on water, fire, etc. Sometimes people straight-up paint on film frames in things like photoshop.
The big win with real-time visualization is the creative control for the direction and director of photography, who are generally far removed from the final product and this can cause expensive second-guessing all around.
Lastly, it's a bit condescending to artists working on games to suggest that the painstaking work that goes into making/optimizing/QCing interactive content is something that can just happen on the fly. Sure, game engines and hardware are pretty great, but games themselves are more and more realistic because very specialized people are working very hard to make them that way.