Provide your evidence please?
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You realise animals raised for consumption are given the supplements for it (B12 for example) to be passed down the chain into humans, who then destroy the nutrient by cooking the meat. I'd rather skip the middle man. Especially when doing so has endless benefits.
In answer to your first question, don't have to, don't want to. I believe you should be free to destroy your own body if you want and that you should also be free to believe all of the hype and marketing you want. Especially when a quick google search will give enough information to question some basic assumptions of the strict vegan crowd.
With that said, my family does regularly eat veggie only meals. But we also eat fruit, dairy, pork, beef, chicken, fish, seafood, etc. Why? Because food research is like global warming, it is complex enough that no one really knows and the research is bought and paid for by various food factions.
With regard to animals, I can find some agreement with you, but being vegan is not the answer. First of all there is no science that manufactured vitamins are beneficial except in special cases (for example minerals) based on bioavailability. In addition, for example, most natural vitamins are much more complex than their manufactured counterparts. IIRC, vitamin K for example has about 35 variants when it occurs in nature, But manufactured vitamins K is only one variation. There is almost no science studying the impact of these variations or lack thereof on humans.
Again, some more reading would give some opinions that not all vitamins are destroyed by cooking. But like you said there is a reason that people should eat some raw meat because it does contain nutrients that humans need and can only get from raw meat. But fairytale minds will never consider that, or even read the research. For example there is no research explaining the reduction of raw meat consumption in the 1900s and the increase in chronic noninfectious disease. Are they cause and effect, or just coincidence? No one knowns. Why? It might be bad for the food and vitamin industries?
Some meat (non organic, non free range) also has added hormones and other problems and I am not aware of any real research that proves they are not harmful other than blanket statements from FDA. We do only eat organic, free range, grass fed meat, when possible.
Don't get me started on the pesticides on and radiation of your much loved veggies. Certainly not good for you. In summary, there is no one food that is good at the exclusion of all others. That is just marketing hype and BS.
I just think that it is smarter to balance my exposure to both good and bad by eating a variation of food and not relying on marketing BS that has been paid for by some food faction. But if you are going to believe some research, then you have to believe all of the research, not ignore research that does not fit a fairytale mind.