But they can and do.
Individual biochemistry is always a factor. Always.
Carnivore and veganism, two opposite extremes, both will have individuals who will thrive.
In my own family -just a single anecdote- I have a vegetarian since the 80s husband in kickass health, a vegetarian -never had meat- now 7yr vegan daughter aged almost 36 who looks like a fitness model, and a good pal who (admittedly is a mutant freak of nature) who is aged 56 and in afreakingmazing condition (is an extreme hiker) on a packaged food/high sugar diet.
I’ve worked in med sci for decades and could cite hundreds of cases such as the above if I went back over charts.
Anyone who states a single diet is best for all is usually a layperson with zero scientific knowledge, an agenda (money to be made therefrom) or both (Jillian Michaels, I’m looking at you).