Here's why:
* The Witches are an organized group: The witches in the book are part of a secret society that actively seeks to harm children. They have specific goals and rituals.
* Daisy is a victim: Daisy is a young girl who is unfortunately caught in the crosshairs of the witches. She is targeted by the Grand High Witch and her coven because she is a human child.
* The story focuses on the witches: The story is about the boy's fight against the witches and the danger they pose to children. Daisy's role is that of a victim, not a witch.
While it is true that Daisy Entwistle has some similarities to a witch in the book (she is a powerful and independent young girl), she is ultimately a victim of the witches' cruelty, not one of them.