I grabbed some advanced diet whoop-de-doo guinea pig pellets for my guys last time I was out. It had a glossy package and promised to make them healthy and happy. Inside were little green, yellow and orange cheerios and various seeds. In fact, I just read the ingredients, of which there are fifty-eight, including three food dyes.
After several days, I noticed colorful little cheerios scattered around the pig pen. They not only didn’t want to eat them, they wouldn’t tolerate them remaining in their food dish. Now guinea pigs are the hobbits of the animal kingdom. They are somewhat obsessed with food (yet another way they resemble humans.) They squeak and whistle when they hear the refrigerator door open or the rustle of a plastic grocery bag (carrots? celery? or-be still my heart-Romaine salad mix?) Pigs that they are, they still utterly rejected the supposedly healthy additions to their pellets. Given a choice, they would live in tall grass, munching all day long with a grass canopy overhead to protect them from predators. But vitamin-enriched cheerios they utterly reject.
Of course, humans buy pet food and pets do not. If pets bought pet food, your dog would choose a pound of fresh road kill, not gourmet meatballs with brown gravy.