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Most magical girls aren’t.
A Magical Girl who fights Witches often dies. This is suboptimal: one who dies before maturing into a Witch wastes the power invested into them. There is a solution to this. Those with vastly more potential than average learn their wish will make them a Magical Girl. The rest get a wish.
They do not get told of Witches, Magical Girls, or the importance of Grief Seeds. They live for days or weeks or months or (very very rarely) years as their Soul Gem darkens unbeknownst to them, and then one day they become a Witch.