One of the few explicit rules of magic in Harry Potter is that food cannot be created from nothing. Specifically, it's one of the five exceptions to Gamp's Law of Elemental Transfiguration. Other things in Harry Potter that are known to create objects out of nothing, like the Room of Requirement are explicitly noted as not being able to create food. The magically appearing food in the Hogwarts Banquet Hall is notably not being created out of nothing, it is being made in the kitchens by house elves and summoned from elsewhere using charms.
Supposedly food can be magically duplicated, but it's never made clear how this differs from creating food out of nothing. One possibility is that duplicating it increases the amount of food but the nutritional and caloric content remains the same, so that all you'd be doing by duplicating food is partitioning your rations. This is never stated but it's the only thing I can think of to explain why duplicating food doesn't violate Gamp's Law of Elemental Transfiguration. One would think that Hermione of all people would know if duplicating food was a safe thing to do.
Also note that Harry Potter magic is notoriously inconsistent even within the context of the story. Food may not be conjurable but wine and sauces can be (as is other potentially edible organic matter), and people living in poverty have been known to survive off of alcohol and ketchup until they can save up enough money for food. Honestly, there's much bigger problems given that it's established that inanimate objects can be turned into edible animals and plants. Given that Harry, Ron, and Hermione's third year transfiguration final in Prisoner of Azkaban was to turn a teacup into a tortoise, why didn't they just use that spell on a rock and have turtle soup with ketchup for flavor and a nice red wine?