Because Montessori specially emphasizes children's learning is spontaneous, coming from free selection, which requires self-responsibility, self-development and the growth supplementing each other. The duties of directress are to "observe children, understand children and follow children", the directress is in fact not "teaching" children. 

Furthermore, Montessori repeatedly emphasizes this is the meticulously prepared "ready environment", so it is called "Montessori Environment", or "Children's Houses" named by Montessori.