All vertebrates are made of "meat," but not all vertebrates could live by eating it.
Imagine trying to feed a diet of meat to horses; within 3 years at the outside they would die of either Vit A poisoning or kidney failure due to the overload of toxic bi-products of proteins as they are metabolised.
Humans are natural omnivores, which gives great versatility in survival...
but the fact that we are partly made of meat, doesn't mean we need meat to build our own. By that reasoning, we would need to eat each other's brains in order to acquire intelligence!
I do believe there are a small percentage of people who do not thrive on a purely vegetarian diet, even if it is continuously well balanced. Not sure why, but suspect it is connected to individuals genetics variants in metabolism.
Not all vegetarians say we shouldn't eat meat.
I have been one for 34 years, and from the beginning have been adamant about not attempting to persuade others to live as I do.
It happens I do think humans would be healthier, with less extreme poverty in 3rd and 4th world nations, and the ecology of the planet better husbanded if all humans ate much less meat - say a cube of 2" per day, unless growing, healing or doing hard work with muscles which require a lot more protein --- But that it only my opinion, and I do not believe I have the right to impose it on others.
There are two ways I veer close to "persuasion."
One is inviting friends for home cooked meals -
- the tastes of the food, the feeling afterwards, & seeing my health
is wordless testimony.
The other is a "story' on my profile page which explains the various different reasons why people choose vegetarianism, and which explains why some are not so strict.
Some people have no option but to include meat in their diet for optimum good health.
Others are better off eating meat because they don't know enough about nutrition to balance their diets correctly, and may also lack the skills of food preparation.
Anyone who tries to be vegetarian on fast foods would end up in a very sorry state in only a few months.
I've also met people who overdid it on seaweed - not a good idea.