MFW people ask about Medieval art
...though I'm being unfair here, as you did say 'oftentimes', but one never tires of bringing out Byzantine Flat Jesus Khristos Pantokrator in response to Medieval art queries. To your question directly, 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?"
