As are dogs. Although many ARE fed an omnivorous diet, said diet is not the best for them.
I feed my dog grain free food. But dogs in the wild will eat a more omnivorous diet and can survive on it. Cats cannot subsist on the same sort of diet, they are what is properly termed an obligate carnivore.
http://en.wikipedia.org/wiki/Carnivore#Obligate_carnivores