IMO they don't process enough of it to make it worth while feeding it. There are all the necessary nutrients in meat, bone and organ. Bone replaces fiber, so there really is no need for plant matter.
Actually, I think that if people reverted back to what "cave" men ate, they would be a lot more healthy. Look at the average American? Obese. Cave men? Not so much. Diabetes? Is a "new age" disease. Etc, etc.
If you look at some of the fad "diets" out there (the good ones I mean...just like a raw diet for dogs/cats) they advocate eating only lean meats, fresh fruits/veggies, nuts and berries...but not grains. Why? Grain is pretty useless if not processed to make breads and flour based foods. You would be amazed at how well you feel if you totally change your diet to emulate what primitive man ate. Of course in this day and age you have to take other things into consideration...like food processing (meats and such) that would make it not beneficial to eat it raw. Look into doing the South Beach diet. I always feel better when I'm on it, compared to when I am not LOL!