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?
Art isn’t a progression. “Realism” isn’t necessarily a goal.
If capturing some subject in the most naturalistic way possible was the goal of art, there would be no need for paintings after the invention of the camera, and sculpture would be obsolete because of 3d scanning. Art must be something beyond capturing nature, otherwise there would be no reason for humans to do it.
For a long time, art was also the only way to portray what people and places looked like. Not all art needs to try and convey the reality, but some ancient patrons would undoubtedly have wanted portraits that looked as close to their real faces as possible. The Fayyum portraits in another comment and other explanations, such as materials being mostly handmade and paintings being unlikely to survive answer my question better, really.