Why can't humans digest all carbohydrates?

The ability of humans to digest any compound is present on enzymes which can break them down. Humans lack enzymes required to break down certain carbohydrates such as Cellulose.

The enzymes that digest carbohydrates are very specific and can only digest specific glucose monomers of a Polysaccharides. For example our body can digest starch since its made up of α-glucose monomers (the difference of α-glucose and β-glucose is just on the location of the hydroxl group). However Polysaccharides like cellulose are indigestible because cellulose is made up of β-glucose and the enzyme which digests polysaccharides in humans (amylase) cannot detect β-glucose linkages.