Question

why humans and many animals cannot digest cellulose

Answer 1

because our digestive system is not so strong to digest cellulose

Answer 2

we humans lack the enzyme that digests cellulose. In fact , all mammals do.  But ruminants, those animals which can digest cellulose (you know, cows , camels and such ) do so by harbouring cellulose- digesting microorganisms in their digestive tract.