Why can't humans digest all carbohydrates?

Humans are capable of digesting some carbohydrates but not all of them. The human body cannot digest carbohydrates such as cellulose because it does not have the enzymes to break down the cellulose into simple sugars. This is not a bad thing. Cellulose is a fiber which helps the intestinal tract to work efficiently.  The human body is designed to digest and absorb nutrients derived from food that has been eaten. It cannot function properly if it is not able to digest the food that you eat. Some parts of the carbohydrates are undigestable to provide fibre and roughage which helps the working of the digestion system.