For example, places like HistoryPorn have some bizarre pictures of weird inventions or WWII experimental weapons.

How come I’m only just now coming across them? Why didn’t we see them five or ten years ago, even in specialized forums and subreddits?

Places like ArtPorn or TraditionalArt are a trickier proposition. Here my lack of knowledge is vast, but I’ve really loved the history of painting for over two decades now, and have recently kept coming across a lot of 18th-to-20th century paintings and painters I’ve never heard of before; some of these are excellent, I should have known about them… I think. But like I say, there’s more that I don’t know than what I do.
If they are real and not recent AI creations, where are the original and who is digitizing and/or publishing so many of them all of a sudden in the past year?