You aren't annoying anyone, this isn't a terrible question.

But, the answer is, why wouldn't they? You cannot apply the rules of human nutrition to other animals. Let me flip your logic for a second.....

There are jungle cockroaches that can run 150 times their body length in one second, and can lift 20 times their body weight, that eat nothing but rotting wood. OBVIOUSLY, if you go out and eat a straight diet of rotting wood, you won't et that strong or fast.

Humans are not good at digesting grass. We lack the enzymes, physical structures (like multi-chambered stomachs and grinding teeth), gut microbes, and lifestyle to get the most out of grass and other tough plants, unlocking the nutrition.

We lack the DNA, growth hormones, sex hormones, bone structure, and life cycle (ontogeny: the study of development and life cycle) to turn into giant muscular tanks, in the same way buffalo lack our ability to live to 60-70 years.