Here's why:

* Cell walls are rigid structures that surround the cell membrane of plant cells. They are made primarily of cellulose, a strong and fibrous material.

* Cellulose fibers are arranged in a network that provides structural support and rigidity to the cell. This network helps maintain the shape of the plant and allows it to stand upright against gravity.

* Turgor pressure, the pressure exerted by water inside the cell against the cell wall, also contributes to plant stiffness.

While other organelles play important roles in plant cells, the cell wall is the primary structure responsible for their rigidity.