Here's why:

* Cellulose is a polysaccharide made up of long chains of glucose molecules.

* It provides rigidity and support to plant cell walls, giving them their structure and allowing plants to stand upright.

* It is the most abundant organic compound on Earth, making up a significant portion of plant biomass.

While other carbohydrates like starch and sugars are also present in plants, they primarily serve as energy storage and are not the primary structural component.