Why did it take so damn long for humanity to "learn" how to draw/paint realistic images?

I mean, you take one look at Greek statues and Roman busts and you realize that people figured how to aim for realism, at least when it came to the human body and faces, over 2000 years ago.

Yet, unlike sculpture, paintings and drawings remained, uh, "immature" for centuries afterwards (to my limited knowledge, it was the Italian Renaissance that started making realistic paintings). Why?