If starch and cellulose are both polymers of glucose, how come people can digest starch but not cellulose?

Cellulose is a structural component of the plant cell wall. It is a polysaccharide consisting of a long linear chain of multiple β - 1, 4 linked glucose units.

Cellulose is difficult for most animals to digest except herbivores because they lack the ability or enzyme to digest cellulose. This enzyme needed is the cellulase which cleave to its β - 1,4 glycosidic bond and hydrolysis it into its constituent molecules (glucose).