I think it is not just lack of dirt that gives that impression.
The shine on the plane is very even. In real life the paint varies in shininess slightly, and there are small irregularities in the smoothness of the wing metal.
We have yet to see a WiP aircraft with part-shiny skin where panel lines and hatch edges don't shine, or where these edges deflect the light in some way. It would be nice if this was possible..but not essential of course.