It's not absurd if it's true. Peoples' dietary needs and tolerances are highly variable; I know people who are vegetarian because they can't eat meat, it actually makes them physically ill on the level of a medical emergency (I suspect a red meat allergy, but I am not a doctor). I don't have a dietary fiber requirement--my optimum level of fiber is strikingly close to zero, and some 4 grams of dietary fiber without a substantial amount of animal grease in a day causes severe constipation. I don't need to avoid plants; I just need to avoid salad.
It's absurd that you think that people on one side of the world have the same dietary requirements as people on the other side of the world; it's still absurd that you think people on one side of the street have the same dietary requirements as their neighbors. In some parts of India, people are largely vegetarians; some of them eat insects as well, which is meat. Neanderthal man required at least 5000 calories per day to sustain, and had an incredibly long digestive tract; caucasian, asian, and negro man are quite physiologically different, and within these groups there are hundreds of variances. Some caucasians--a group largely raised on dairy, i.e. Europeans--are lactose intolerant by some damnable magic.
Those fallafels and rice cakes and red bean paste dishes and sweet potatoes are all nutrient rich, even protein rich, but they don't manage to give enough of what I need in a way that I can absorb it. Bioavailability of choline from soy is exceedingly poor--lack of choline will stunt neural development and reduce the amount of brain activity you can sustain. B12 is extremely rare in plants, but common in meat. Amino acids are readily available in meat, but they're available in different amounts and in different protein chains in plants--chains that don't always break down as effectively as those in meat. Fat is hard to find--avocados have plenty of it, that's about it.
The point is that "all the pieces are there" in the same way that all the pieces of a house are in the house next to mine, which was just demolished after tearing them down. Bricks, lumber, and mortar are readily useful; however I would have to scrape mortar from the bricks from the old house--doable--and use chemical resins and reagents to process wood and mortar into useful material--I'm not equipped for that. And of course much of the material is damaged (burned wood, contaminants, etc.), so I can't access it in any useful way at all.
Your haughty idea that we can slop the same nutrient-rich gruel in front of anyone and expect them all to grow up equally as healthy with no deviant impacts from their diet is pure delusion. It doesn't match with science, it doesn't match with anecdote, it doesn't match with the world around you if you stand back and look for a minute. You may as well claim the world is flat while you're at it.