As others have said, I believe the lack of depth in medieval art is much more a matter of conventions than a matter of lack of theory or physical abilities (see [0]).

The one thing that convinced me of that is the sudden jump in realism in Egyptian art during the reign of Akhenaten (Amarna period, see the famous Nefertiti bust for a great example). The new king asked for more realism and he received it, it just took a reset in conventions.

[0]: https://en.wikipedia.org/wiki/Perspective_(graphical)#Early_...