I'm not an art historian by any means, but you may be interested in some of these previous answers from our FAQs:

https://www.reddit.com/r/AskHistorians/comments/1hqcvq/why_were_sculptures_so_well_done_in_ancient_times/ (24 comments, two years ago)

https://www.reddit.com/r/AskHistorians/comments/1ceool/why_couldnt_people_draw_realistically_in_the/ (12 comments, two years ago)

https://www.reddit.com/r/AskHistorians/comments/1l1vp2/why_is_ancientmiddle_ages_art_is_technically_much/ (15 comments, two years ago)

https://www.reddit.com/r/AskHistorians/comments/199hmy/how_correct_is_the_view_that_we_lacked_the/ (5 comments, two years ago)

These are all from this section: https://www.reddit.com/r/AskHistorians/wiki/art#wiki_why_wasn.27t_ancient_art_realistic.3F

If I could summarize the arguments very briefly, it boils down to this: Art has cultural meanings that go beyond photorealism (and it would be interesting to contemplate the effect that photography has on "realism" in portraiture). Much art is based around telling a story, and many stories aren't best told with a full-perspective 3-d view but can be better told with sprawling and even non-linear depictions of scenes.