I, for one at least, consider along the millions of years that evolution tried a version of homo sapiens that couldn't eat meat, and it was selected against...why would we now attempt to overturn the apparent wisdom of that selection?
That's pretty much a no-brainer for our health, for the environment and to reduce the suffering of millions of animals at the hands of the industrial livestock industry.
Personally, I used to eat meat multiple times a day, and now I only eat it once or twice a week. It has really improved my life. I think everyone should try it.
Because it doesn't scale."I've been disrupted by a leaner competitor!"
Our bodies specifically evolved for reproduction, but that doesn't stop us from using contraception when we have sex. Evolution also designed us to have wisdom teeth, but that doesn't stop people removing them.
Your argument assumes that natural selection has some kind of innate "wisdom", and that's just not true. Natural selection doesn't produce perfection or have any kind of intelligence behind it, it simply produces "good enough" (or, as we're on HN, the minimum viable product).
Also it's probably a mistake to attribute something like wisdom to what is essentially a statistical crap-shoot. The great temptation of religion is of course the idea that there is an Intelligence at work. But just to pick a silly example, does the dodo bird or the passenger pigeon think it was "wise" and "intelligent" for the forces of nature to select for meat-eating humans? Dodo philosophers and theologians in the last days - did they question why God would unleash such a fury on them?
Anyway I can tell you human ones will. Natural selection will most likely start killing off humans as soon as our technology can't keep up with all our baby-birthin'. It will manifest in forms you're already seeing in today's headlines. Voluntarily consuming less is an attempt to deal with it before the wisdom of natural selection deals with it.