What is not to understand?
Cow is adapted to eating grass, from its teeth to its digestive system, all highly evolved to be a grass feeder.
So evolved in fact, that eating and digesting meat would be near impossible for it.
Human is evolved to being omnivorous, and gaining certain things from meat.
Our teeth are poorly suited for eating grasses, and many harder plants.
They are well designed though for eating meats and softer plants.
Take a good look at human teeth, not super terribly efficient design for heavy plant eating are they?
We require B12, and yet we can not get it directly, we can not eat the things that carry the bacteria that produce it.
We have adapted to get that by eating the meat of those who can.
We evolved to be able to eat something virtually any hospitable place on the entire planet, and that includes meat.
We require proteins that are not just laying about naturally in the plant world.
It may be shocking but peanuts etc don’t exactly just appear all over, and other sources are seasonal and tend not to be readily available in mid winter, but meat is.
We also actually need fats and other things that are in meat.
It isn’t something some modern guy decided to start a trend in, it is how we adapted and survived.
You don’t just wake up and say Hey, i am modern man, i am greater than nature, we no longer eat meat, or berries, or roots, or what ever thing you decide against.
You cant do that no more than you can wake up and decide man no longer walks on 2 spindly legs, we are going to fly, or run on all 4 like a horse.
We did not evolve around a modern grocery and food supplement store, and we spent over a million years getting to and perfecting what we are.