The issue is that there are 500 different kinds of vegeteriansism/veganism/omnivorism/carnivorism.

So if a certain person eats a slighty different variant across his lifetime you could easily have one person become deficient in X but the other one won't even though they both follow "roughly" the same diet (eg vegan).

Also the extremists in each diet camp are the loudest ones always, and those extemists will be the last to see the forest for the trees (eg how a slight variation could be beneficial).

In general no one knows what the best diet is.

Up until ~100 years ago we just needed food to SURVIVE. Now the question becomes: once we have all food available 24/7: which is the best food to THRIVE on?

The body isn't that good when it comes to detecting this, eg if we'd let the body do its thing it would just eat cookies all day long..