Why are all humans not vegetarians and vegans?

This documentary (https://www.youtube.com/watch?v=32IDVdgmzKA) shows what a horrible life animals that get turned into meat have on farms.

I think if all humans were vegetarians and vegans, this world would not only be a better place for animals, but also for humans too.
Update: And also this website (https://www.huffingtonpost.com/kathy-freston/shattering-the-meat-myth_b_214390.html) and this website (https://blogs.scientificamerican.com/guest-blog/human-ancestors-were-nearly-all-vegetarians/) explains why all humans should be vegetarians and vegans.
Update 2: And also this website (https://www.livestrong.com/article/2898-facts-lactoovo-vegetarian/) shows why a vegetarian and vegan diet is better for humans than a meat eating diet is.
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