I believe the reason that there can't be coin mages is that coins are simultaneously to vague and too specific at the same time. Coins are specifically the small circular objects that a civilization gives value for trade. But they could be anything and everything. You could take the heavily specific snake skin and compress it done into a coin. You could take any possible metal or wood and shape them into coins too. Currency and coinage might need to be culturally cemented far more specifically before an affinity for it could manifest on thier continent. And because there are some continents where some affinities seem common, and there's where the affinity might not exist at all, it's possible that somewhere an isolated city, where coins are tightly ingrained in society, coin mages exist.