Here's why:
* He is possessed by Voldemort: Quirrell's true purpose is to obtain the Sorcerer's Stone, which is hidden in the last chamber of the school.
* He's acting as a guide: Quirrell acts as a guide for Harry and his friends as they navigate the challenges set by the Sorcerer's Stone.
* Voldemort needs the Stone: Voldemort, inhabiting the back of Quirrell's head, wants the Sorcerer's Stone to regain his physical form.
Therefore, Quirrell's role in the story is to *guide* Harry to the chamber where the Stone is hidden, not to be in a separate chamber himself.