We get way more from plants than just fiber. Proteins, fats, sacharids, vitamins, antioxidants, minerals including calcium. Our ability to digest vast diversity of plants and their different parts is rather astonishing. Maybe you mean our inability to digest grass (and also cellulose in general). We're really not good at that, unlike e.g. cows or sheep, who have specialised digestive tracts with symbiotic bacteria that help them, so they can feed on grass alone.