Different vegetarians have different motives. Some of the more common ones include:
Moral concerns, e.g. about animals suffering or being killed. This is common among Buddhist vegetarians, animal-rights vegetarians, and utilitarian vegetarians.
Health concerns; belief that a vegetarian diet is better for one's health, whether due to substances naturally in meat (e.g. saturated fat) or introduced by industrial meat production.
Environment and climate concerns; that raising animals for meat is bad for the environment, contributes to climate change, is unsustainable, etc.
animals are fed parts of plants that people can't or won't eat. all of the studies about the ecological impacts ignore this fact and then attribute the water used to produce, say, cotton to beef.
Today in the developed world, meat animals such as cattle and hogs are mostly fed fodder — not a byproduct of human food, but rather crops grown specifically as animal feed. Farmers who grow those crops know how much water they're using, because they keep track of their irrigation needs.
Just how many times did you copy-paste that comment?! Are you a bot or a lobbyist by any chance?
You think that we started producing some grains, and one day we realized we had too much by-products and one smart guy said: "let's start a cows herd so that they'll eat these". Sounds legit. Especially if you consider that eating beef the way we do is very recent in human history, and still inexistent in many parts of the world. Poor folks must be buried under the by-products...
So, since I don't think farmers are total morons, I would rather imagine they would produce different kind of food, such as leguminous.