The subject came up in an /r/MedievalCats post where people were asking "has this dude literally never seen a cat?" A copy of my response is in the comments. What am I right about and what am I missing?
Art styles change. Realistic depictions wax and wane in popularity.^1 More can always be said on the matter, but this section of the FAQ is most appropriate, specifically the 'Medieval Art' subsection, in particular the answers from u/silverionmox and u/Guckfuchs.
1 - Look at Guernica and you may yourself be moved to ask, "Has Pablo Diego José Francisco de Paula Juan Nepomuceno Cipriano de la SantÃsima Trinidad Ruiz Picasso ever seen anything in his life?"
Context:
So I'm repeating things I learned in European/Art History classes that I honesty still have a hard time grokking, but best I can tell is nonetheless true:
European artists, as a culture/discipline/institution, "forgot" how to draw things well after the Classical period. The collapse of the Western Roman Empire also meant that the traditional sources of patronage for artists dried up. With less money/focus going towards art, there were fewer professional artists, both as teachers and as students, and less opportunity for a creative type to, well, create instead of working the land.
As a result, some techniques stopped being passed down from artist to artist (why I use the word "culture" originally). When artistic patronage came back, those techniques had to be "relearned."
It's helpful to conceptualize by thinking of cave paintings. Biologically, early humans were more/less identical to modern humans, so were theoretically able to make the same dexterous hand movements and accurately observe the world around them, yet their art looks like that of a contemporary child.
One of the specific techniques that was lost was an understanding that adult human faces are fundamentally different than others. Artists learned to draw adult human faces as "how to draw any face"--the cat's mouth looks inaccurate not because the artist had never seen a cat, but that was just the best he could do based on how he learned to paint. You can see the same thing with Renaissance paintings of children and babies--95% of the painting looks near-realistic, but inexplicably the child has an adult face.
One final confounding factor, creating "art" just for aesthetic value is a very modern convention. Before, art would be commissioned for a specific purpose--i.e. inspiring awe in God or demonstrating the power of the state. Therefore, lots of European historical "art" is less art as we know it and more marketing/propaganda.