@dolmansaxlil Muffin, when teaching kindergarten and first grade, reported the same anecdotal observations as you.
As for short stories, my thoughts on short stories as both a writer and a reader are also anecdotal and subjective. I think science fiction works very well in short forms. I read a ton of science fiction anthologies when I was a kid. Some science fiction stories really are about a very cool idea that's best told in a succinct, straightforward way. Expanding them beyond that idea feels like padding, and the extra material detracts from the punch that comes from delivering the answer to the story question and quitting.
To me, fantasy requires worldbuilding that doesn't work as well in a short work.
Romance can work, but you lose the ups and downs ot several rounds of will-they-or-won't-they and/or they-did-but-was-it-a-bad-idea?
I have to believe that mystery short stories can work, since I write them. Editors give a lot of latitude for mystery shorts, generally requiring that the story involve a crime without requiring that it be a play-fair mystery that gives the reader all the clues. I, however, prefer my stories to have clues and a logical structure, so that's what I write. I find that it's hard to pull that off in less than 5000 words (~20 manuscript pages) and I prefer to have at least 6000 words.(~24 manuscript pages.)
Literary short stories are their own thing. When I like them, I really like them. When I don't, they feel plotless and self-indulgent. I really admire George Saunders and Elizabeth McCullough when it comes to literary stories. When it comes to (sometimes fairly weird) flash stories, Lydia Davis more or less made that arena her own.