Why is QB called a slut in the small circle of witches?

I haven't found the reason why QB is called a slut in the small circle of witches. But I can introduce you to the setting of QB in "The Witch Circle".

In the small circle of magical girls, QB is a creature who collects the desperate energy of magical girls for a living. The condition for them to gain desperate energy is to sign a contract with a magical girl and realize one of their wishes.

If you need more accurate information, you may need to consult relevant materials or consult Japanese professionals.