The countries with the highest grain consumption are most often the most malnourished too
Part of the reason for that is that grain is cheap filler food. The double-edged sword of moving from hunter-gatherer groups to agriculture-based societies is that, yes, population increased and other aspects of human development came about, but the people were smaller and weaker. In highly populated areas of Asia where rice is consumed, it may not be feasible to incorporate meats other than some chicken, fish, and pork.
But then there are areas of the world (many African countries) who switched from subsistence farming and very little famine to “modern” monoculture farming, only to find it is not a sustainable model. Once traditional farming is gone and the populations are dependent on grain, then a single crop failure can be disastrous; monoculture also has the distinction of stripping soil of nutrients (and allowing topsoil to be washed away in general), so even without crop failure there are diminishing returns. (In the US, farmers compensate with expensive chemical fertilizers, herbicides, and pesticides that third world farmers may not be able to afford.)
Another part of the puzzle is that when grains were prepared using traditional methods (fermentation, sprouting, etc.), they did offer up more nutrients. But as Western methods of food preparation accompanied our “superior” farming, many of those food traditions were pushed aside. Sadly, the US does supply food to much of the world, not so much like a knight in shining armor, but more like a drug dealer who gets his clients dependent