Saw someone on Reddit who said that their mother would keep leftover food sitting out on the counter, un-refrigerated. She couldn't understand why it was growing mold several days later. In her mind, "cooked" meant "safe from spoilage".
Jokes aside, I think it's simply because grapes are so sweet and naturally high in sugar. And then you add more sugar for jelly-making; it's a bit gilding the lily, I guess. Grape preserves seem to be uniquely American, like cheese in an aerosol can.
https://www.fodors.com/community/europe/no-grape-jam-in-france-961126/
5,000 years ago, Ancient Egyptians would treat infected wounds by applying a poultice of moldy bread. Nobody knew why, until 1927, when Alexander Fleming made the association between moldy bread and Penicillium, the type of mold from which modern penicillin is derived.