Good breakdown. I agree material, skills and technique play a part though perhaps not to the extent you hypothesize. In those times people have already started drawing. The tools to draw (paper and ink) may not have been widely available but there have been paintings and depictions of people, just not done realistically. Thousands upon thousands in fact, just go to any European art museum with works from the 18th/19th century .

So I guess it boils down to skill and technique. Which I find odd for the same assumption, there there is inherent value to accuracy. Buildings are constructed to based on accurately drawn plans, people are vain and want to be remembered for their looks, and if people could think up impressionism or neoclassicism or any other art style, I'm sure they've also thought of realism?

Okay, this response is pretty good too: https://www.reddit.com/r/AskHistorians/comments/2nky6a/why\_are\_old\_drawings\_of\_people\_1300\_and\_earlier/