Weren’t there artists at that time who could draw humans for what they actually looked like. For instance, look at the paintings of kings from the 17th Century or before.
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Its all about how we pass the technic on. Painting is mostly technic, from proportion, light, perspective, everything a modern painter or illustrator does he learned. If the painter had to “discover” these technics he would die before painting something realistic.
Ancient people had to develop these technics sometimes from scratch. Sometimes the technics developed over centuries just get lost, and people had to develop all over, sometimes with very strict rules (witch could impose less realistic texhnics in favor of a more abstract way of representing)
Let’s take the classic greek statues as a exemple: the Greek had a stable society where artists could explore and develop technics, coming to a high level of realism. When the Greek empire fell, the realist technics gone for good. Until two thousand years latter, when the renascentists artists had resources to dig for greek statues and reverse engineering, and thats why we can withness a lap in realist technic in renascence.
Important: Your question is misleading, because there are realistic paintings from ancient civilizations, from Egypt to Greece. Also, painting is related to a more eurocentric tradition, sculpture was the most desired form of picturalization in most cultures. “Why paint your wall with a ugly 2d face (lets remember, a canva is a very specific medium) if you could have a 3d model?” I guess: from cave man the carving was more present than the painting, as a way to represent the world around. And we have A LOT of really technical and realistic sculptures in ancient cultures from all around the world.
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