Fridge: The Miko's Words and the Witch's Incantations
Fridge Logic: Why doesn't the mountain god transfer its powers into Tsumugi herself?
Well, presumably It takes an extremely strong constitution in order to be able to handle a spark of divinity in a human body. Not purity, but a type of spiritual toughness and adaptability. Even the Mountain God comments that it's possible that it would kill Letty.