why do raw vegetables act as a roughage in our diet but serve as food for herbivores

raw vegetable like tomato is digested by us but our digestive system cannot digested leaf, grass because our gut does not have microbe to breakdown it etc. Most herbivores have four chamber stomach. they have microbe in side gut to digest it. so after eating herbivores do a process called " chewing the cud". Fibres in vegetable food called roughage are not digested by us.