Actually it's the other way around.... think about it.... dogs and cats seem to be interested in consuming their own feces or that from another, they drink out of mud puddles and stagnant water, they gobble down pure fat from meat, eat out of day old, fly-infested garbage, etc., etc. and through all that they seem to digest things fairly well most of the time. So who has the stronger and more tolerant digective system? Not us humans!
Actually believe it or not, our digestive systems are fully capable of digesting that kind of crap; its our disgust instinct prevents us from eating that sort of sh1t.
Our stomachs can digest all types of soft animal tissue including fat, skin, cartilage connective tissue etc. Some people in the third world eat insects, fat, raw meat, rodents, maggots etc.
There are people have virtually no veritable matter in their diets: the inuits survive the long winter on a pure meat diet, getting all the vitamins they need from animal tissue.
At the other end of the spectrum there are vegans who refuse to eat anything of animal origin.
We are omnivores and can eat just about anything, cats and dogs are purely carnivorous and are restricted to meat. Our systems can digest both meat and vegitable matter.
Unlike dogs and cats our systems can also handle vegetables which contain many chemicals not found in meat.
I was comparing the liver more than anything else: our livers can breakdown all sorts of strange chemicals not found in meat.
Rats are omnivorous, like us. In fact they're better omnivores than us, they can eat all sorts of plants that are poisonous to us as well as most of the foods that we can eat.