because it's cheaper, easier for people with less knowledge, and people buy into fancy smancy packaging.
Purina puts giant pictures of meat in the advertisements for dog food, and you'll be lucky if a meat product is even listed in the first 6 ingredients. Dogs are carnivors, they can't handle more than 25 grams of carbohydrates a day, yet they are being fed around 80 to 100 grams of carbs. Then people wonder why their dog is going blind, or is fat, or has diabetes and a dull, stinky, itchy coat!
It's the same crap with people, have you ever left a package of oreos out on the counter and noticed they never go bad? Not only that, but Ants or other bugs don't even recognize them as food! Real Salmon isn't even pink, it's a light tanish/mauve color and it's died and processed be pink, Tuna is processed with Carbon Monoxide to stay red! The FDA allows over 3,000 artificial ingredients and preservatives to be added to our food without being marked on the bottle. So, if you're trying to save money buying canned food, you are ESPECIALLY being targeted by this poison. People drink milk because the dairy industry funds tests to tell us that milk has calcium and is good for us! Sure milk isn't bad, but it's filled with fat, even the fat free milk still has fat, and it keeps us fat, just like milk is supposed to... because it's meant for a 400 pound baby animal! All of our meat is processed ,chemicals and preservatives are added, and most of our vegetables in the U.S. are genetically engineered and covered in pesticides. I am sorry to say that some people on this board have no idea what a cantalope even looks or tastes like that hasn't ben engineered or sprayed, because you can hardly find them anywhere, even whole paycheck (oops, I mean wholefoods) has genetically engineered food because it can still be certified organic.
Anyway, this happens to horse feed too, because it happens in every other industry. That's why I rarely feed anything besides the best quality hay I can find.