Okay, I have a Star Trek theory. You know how all Star Trek characters are obsessed with 20th century or earlier art? The reason is AI, at some point there are no people making actual art, since AI can't create anything, it just recycles stuff that it knows over and over. There were years and years where no new art was created, so art never moves forward.

OR

I'm high.

@RickiTarr it's a neat thought, we do like it. note that in canon (ever since the first ep of TNG explained it), the humans in Star Trek spent much of the 21st century killing each other and intentionally destroying all culture and also all records both historical and contemporary-to-them. so that's why humans in the Federation era are so appreciative of diversity and culture, they're yearning for the lost greatness that their ancestors deliberately got rid of
@RickiTarr it is also a convenient excuse to not have to invent future art history, of course. Star Trek does a lot of future history but its fictional art is mostly limited to whatever precious little weird wine glasses the props people were able to find