Why we cannot digest cellulose as cattle do?

Cellulose is the major component of cell walls of plants, like vegetables, fruits, and grains in our diet. 

We can’t digest cellulose, due to the lack of the digestive enzyme (cellulase) which is required to break the bonds between the monosaccharides of cellulose and release the energy-rich glucose, hence we are unable to digest the cellulose.

After all, cellulose is necessary to our diet because it is an excellent source of fibre.