Why can't humans digest all carbohydrates ?

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.