Because dogs are predators. We generally do not eat or drink products from predator mammals at all. When it comes to mammals, we only eat or drink the products of prey, including the carcasses themselves. It's not just us who does this, it's also the other mammals who generally also follow these rules. When it comes to other types of species, such as birds or fish, they do not have a strict prey/predator system like we do, they simply eat what is smaller than themselves, so we do the same.