Well, considering the recent update as to why Kalista got a Blood Moon skin
"We liked the idea that the story of Kalista is known all across Runeterra... but different cultures may tell her story differently, as well as have a different idea of how she looks, since most of them - the lucky ones, anyway - have likely never actually seen her.
For example, how the people of Freljord tell the story and picture her may be quite different to the people of Bilgewater... And perhaps in some Ionian versions, this is how they might portray Kalista.
Look at the way the idea of dragons is different in different cultures in our world, though they all come from a similar mythic place.
So that... and it looks cool.
Hope that helps!
Cheers!"
-Ant in Oz
I Imagine that the reason Thresh and Elise are gaining them as well are for the same reasons. You live in a faraway place and hear of this reaper from another land, what does he look like to you? Or a mystical 'spider-queen' who recruits followers to visit their 'god' only for him to devour them as she gains immortality. To me, considering my own region, if I had heard these legends, I too would completely imagine them to look something different