In years past, mannequins were made of wood, plaster and other heavier materials. As technology progressed, lighter-weight yet resilient materials made fiberglass mannequins the standard. The advent of fiberglass allowed for the mass-production of display mannequins that we currently see. Display mannequins are available in all shapes, styles and sizes... from abstract mannequins painted in all white, to highly detailed, realistic mannequins complete with musculature and makeup.
The use of fiberglass allows for more detail where it's wanted, as well as making the mannequins simpler to assemble by being lighter weight. It is not as resilient as a piece of solid wood, but certainly stands up well to the relatively minimal abuse that most department store mannequins experience. They are not unbreakable, however. Fingers and even the tips of noses are sometimes broken if some care is not taken in handling.