We don't convert fiber into fat like herbivores (gorillas) or even omnivores like bears. And metabolically we require a lot of fat, especially due to our very large brains, so we have to get it from somewhere, because we aint getting it from plants.
Also, aren't our bodies structured like predators? We are small, agile, and fast. We have eyes in the front for good depth perception. And our ears are good for precisely locating things by sound.
Being smart we can overcome physical disadvantage by cleverness. You don't need cleverness to compete for plants and propagate the species, especially if you are a herd animal. However, amongst predators, I can see cleverness as a big competitive advantage. Cat and dog owners can observe this cleverness and learning ability in their little predators. Predatory birds also often exhibit greater cleverness than other birds.