There are many degrees to carnivores. They need vegetables as well as meat to provide the correct metabolism and pH of body fluids. Protein or more importantly amino acids provide a variety of jobs within the body. Meat is the best source of protein. Vegetables also provide protein, vitamins, minerals, essential fatty acids, fiber, etc.
Cats can eat more meat then dogs because they have a different body pH requirement.
There are other anatomical differences too. The bottom line is dogs and cats are carnivores and they need meat in order to survive and feel well.