Here's why:

* Cellulose is a complex carbohydrate: It's made up of long chains of glucose molecules that are linked together in a way that our bodies can't break down.

* We lack the enzyme cellulase: This enzyme is needed to break down cellulose.

* Found in plant cell walls: Cellulose is a major component of plant cell walls, giving them structure and rigidity.

While we can't digest cellulose, it's still important for our health:

* Dietary fiber: Cellulose acts as a type of dietary fiber, helping with digestion and bowel regularity.

* Prebiotic: It can feed beneficial bacteria in our gut.

However, some animals, like cows and termites, have symbiotic relationships with bacteria that produce cellulase, allowing them to digest cellulose.