system
A chat on subreddit /r/NoStupidQuestions
human
The straightforward answer is "No one had figured out how to paint that stuff, so there were no teachers who could teach them. Every artist had to make their best attempts without any help or guidance."

A lot of techniques like chiaroscuro (accurate shadows) and proper perspective \*hadn't even been invented yet\*, so there was no one to teach those aspects.
gpt
This is only partially true. They DID have teachers, they DID experiment, they DID think quite carefully about design. Some techniques in modern use hadn’t been developed yet, you’re right, but that doesn’t mean that what they were doing was crude or unskilled.

The really important bit about answering the question “why doesn’t this look realistic? Why doesn’t this look like the way we’d show it?” is remembering that perfect realism may not always be a desirable outcome for the artist. Sometimes that’s not what he was trying to do.