This question gets asked here all the time:

https://www.reddit.com/r/AskHistorians/wiki/art#wiki_visual_arts

These threads in particular might be of interest to you:

https://my.reddit.com/r/AskHistorians/comments/35do0x/why_do_people_in_medieval_art_look_bored_or/cr3jaff/

https://www.reddit.com/r/AskHistorians/comments/1ttglu/why_did_it_take_so_long_for_artists_to_get_to/cebgxjy/

https://www.reddit.com/r/AskHistorians/comments/5mtzw3/why_do_professional_paintings_look_photorealistic/dc7nbik/

The short version is that photorealism isn't always the goal of art, and that what was valued in different places around the world in different time periods was, well, different.