Just to add another factor to the ongoing discussion: artistic talent isn't uniform and never was. Just because only/mostly "immature" art survived from a certain century of human history, doesn't mean that there literally was no realistic art present at the time. Since you mentioned the statues already...

These are from the same era (around 200 BC), but as you may have guessed, made by different artists =P The statue is called The Dying Gaul by the way.

As for painting examples, I guess the Rothschild Canticles^1 book illustrations represent best what most people nowadays would call medieval art. Not exactly realistic, a little goofy ... perspective? Never heard of it. Proportions? Who cares. And who needs shading anyway?! As long as you can still distinguish a human from a cupcake, it's "eh good enough".

I guess that was also what you meant by "immature" art, because it is the same art style as those goofy weird pictures of knights fighting giant snails and rabbits riding cattle into battle and the like.[^2]

That book is dated to be around 1500–1520 so it would be easy to assume that people at the start of the 15th century didn't have a realistic art style yet. But you know what else was made in that same era?

The Mona Lisa (1503–1506).

One dorky meme-esque style, and one realistic, modest and easy-on-the-eyes style in the same century, probably even the same decade. But they were used by different artists.

Now you might be thinking that those art styles might have been intended for their respective purpose or something along the lines: that the goofy, simple art style was used for nothing but amusing little pictures, and the more realistic style was for "proper" art, because noone in their right mind would spend 100+ hours painting highly detailed nonsense just for sh*ts and giggles, right?

May I introduce you to Joseph Ducreux?[^3]

I guess most of you will have seen that meme by now, but this is a real painting made by a real artist - and it is far from the only one. Ducreux created an entire series of similar self-portraits in ... unusual poses and situations.

... so yes, at least that one guy DID indeed spend dozens if not hundreds of hours (plus material costs) painting amusing nonsense for his own entertainement. He was, in a way, the victorian era equivalent of a shitposter (and I mean that in a good sense!)

Long story short: one can't just claim that "they didn't have X art style in Y century" because the truth is much more facetted than that. It is way more likely that each and every era of human history has had people with insane talent who were able to create art as realistic as possible with whatever tools their lifetime had to offer, and also a bunch of "eh good enough" art or stuff that was deliberately stylized for fun. How we percieve said art today depends mainly on what artworks have survived up until now, and/or how popular the surviving art is. (Everyone and their grandma knows about the Mona Lisa, but how many of y'all knew about the Rothschild Canticles?)

If we don't know about any realistic art from a certain period of time, it doesn't automatically mean that there was no realistic art. It may have been lost, forgotten or it exists but it's just not popular enough to be well-known.

[^2]: https://imgur.com/gallery/medieval-marginalia-dump-bKY5h just some delightfully awkward examples [^3]: https://en.wikipedia.org/wiki/Joseph_Ducreux