[epmaybe]

I think the main reason we can't break down cellulose is because we don't have the enzymes to do so. It's not as if cellulose is actually all that different to polysaccharides we are able to digest, it simply has beta 1->4 glycosidic bonds [1] between the glucose molecules instead of alpha 1->6 glycosidic bonds that are digestable by humans.

edit: upon some more skimming of the review article, even with the enzymes we may not have the conditions in which cellulose could be degraded, hence the "four stomachs" and other methods.

[1]: https://en.wikipedia.org/wiki/Cellulose