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.