Basically, because those special enzymes (and the gut mechanisms to make use of them) are a trade-off. It takes a long time and a lot of digestion to break down tough plant materials like cellulose into stuff an animal body can use. Herbivorous animals have long, complicated digestive tracts, especially the ruminant (four-stomached) ones like cattle and buffaloes, and they spend all day long grazing, chewing, fermenting, and re-chewing stuff like grass to make it useable. (Some don't have these long digestive systems and instead rely on easier plantstuffs like fruits and leaves, but then, those are the kinds of things that we can eat too.) This is worth it for them, evolutionarily, because it gives them access to a semi-exclusive food source.

Humans, along with other omnivore/carbivore species, have opted for a different strategy. Instead of investing a bunch of our time, energy, and body mass into specialized plant-digesting equipment, we developed stuff that makes us better hunters and more discerning gatherers. We can't eat grass, but we don't have to spend 12-18 hours a day doing it, because we can just climb a tree and eat a banana or catch a squirrel instead and get the same amount of nutrients. Obligate carnivores like cats have really simple digestion and can't digest any plant material, but they can sleep 18 hours a day instead. Dogs are slightly more carnivorous than us, and will supplement their diets with plants where necessary, but still mostly need meat. We humans are juuust about herbivorous enough to get by on plants alone if we choose to, but we have to be careful about it, and usually fare best with some animal product supplementation.