Why do we have no enzyme to digest cellulose?

Question

As we know, cellulose is the most abundant polysaccharide in nature. Why don't we have an enzyme to digest cellulose?

Answer 1

Most animals that eat cellulose use symbiotic bacteria that live within their gut. However, these bacteria take a long amount of time to break down the cellulose, which is why animals like cows have multiple chambers and an overall large stomach. Humans, who do not have large or multi-chambered stomachs, won't be able to provide the time necessary for these bacteria to do their job properly, and similarly do not have the enzymes themselves to do this breakdown. Since people can get just as much energy from eating meat or from other sugar sources, cellulose is simply not a necessary part of our diet, and therefore we have not needed cellulase to survive.