On the facts, cows are grazers with incredibly robust digestive systems. They're too lumbering to hunt, but they're biologically prepared to extract nutrients from pretty much anything that ends up in their mouth while they're eating: grasses, insects, carcasses, rodents, etc. Ultimately, feeding them animal byproducts wouldn't be viable or cost-efficient (ugh) if that were not meaningfully the case.

As others are noting, it's the wacky industrial modernity doing its grossly myopic hyper-optimization thing that's the problem (and not just in animal husbandry or agriculture...).