The distinction between "art" and "pornography" is also somewhat artificial. It's hard to argue that [3] was not meant to be prurient when the model (underage, by today's standards) shortly thereafter became a mistress of the King of France, based on him having seen the painting. And some stuff that was painted might get you banned from OnlyFans even today, e.g. [4].
But the problem, it seems to me, is that the internet has turned into a place where (1) everything has to be "safe for children" and (2) said safety standards are defined (through US influence, I strongly suspect) to be highly permissive of violence, but super strict on nudity.
[0] https://artsandculture.google.com/asset/the-war/CwHM2HdTO3l2...
[1] https://www.wikiart.org/en/max-ernst/the-angel-of-the-home-o...
[2] https://www.wikiart.org/en/matthias-grunewald/the-crucifixio...
[3] https://commons.wikimedia.org/wiki/File:François_Boucher_-_B...
[4] https://www.wikiart.org/en/jean-honore-fragonard/girl-with-a...
#3, #4 would just seem out of place because of their nudity. Most sane people would realize it is art but it would still seem weird.
Part of the US's double standard involves being exposed to stuff like the crucifixion and the concept of torture at a very young age. The concept that nudity can be appreciated or even expressed outside of a sexual context is not really a thing in the US. The issue is that the only use cases for nudity in movies & TV is going to sex related 99% of the time because if you are going to show nudity you might as well use it to suggest or depict sex.