Welcome to The Second Apocalypse, greatbigsky. Or posting here, anyhow

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I think the opinions offered so far miss some crucial bullet-points of interaction and balance of power. Wilshire almost raises it but then segues with a joke.
The Scholastic Wars have been fought in Earwa and humanity's Few
won. Moreover, the only people who own Chorae are caste-nobility as Somnambulist suggested.
Though we know very little of the actualities of the Scholastic Wars, I'd hazard that in Earwa's historical notation consequences of the existence of the sorcery is a common fulcrum. We have the Shriah calling a holy war against the Schools, which marks the only point of an organized attempt at fielding Chorae against Sorcerers in the current age of Earwa.
I figure there was no generative moment for the advent of armor for Schoolmen because the Schools have only fought eachother, otherwise. Though there might have been some attempts during the aforementioned wars...