Cats and dogs are carnivores, so thier physiology is different. They can aborb the vitamins and such that they need directly from the herbivores that they naturally eat. (Most pet foods nowadays put these kind of things into the foods as extras, due to the high level of animal byproducts thay use to make the food.) But to say that they don't eat veg isn't entirely true. Both cats and dogs will eat grass to aid in the cleansing of their digestive tract. (which is part of the reason cats puke it back up again and leave it for you to step in in the wee hours of the morning)