They are povery food. Archaeological records show that the shift to agriculture coincided with a dramatic reduction in average height, smaller skulls (and brains), rotten teeth, & under development resulting in crooked teeth & other deformities.

Similar things have been seen in modern times among tribes in Austrailia & Africa that were hunter gatherers until just the last few generations. The most recent generations are now sick & obese.

There has been awareness of these things in some circles for a long time. Writings fron the 1800s discuss high fat, sugar & grain free diets being the solution to epilepsy & other serious issues. But since the Great Depression the US govt has been subsidizing sugar & grain production & they created dietary guidelines to support their subsidies. The companies that benefitted from the subsidies funded all sorts of "research" to support their products. Processed garabge tends to have a much longer shelf life, lower shipping costs, & higher profit margins than real food.

The inflation of the 70s exacerbated problems by driving costs up. Real food got more expensive. Processed garbage was further cheapened with veg oils made from crop waste (previously used as machine lubricants) rebranded as affordable food ingredients.

Modern farming practices & genetic modification of crops (to make them resistant to pests) make grains & soy even more harmful to human health.

But farmers have always used grain to fatten livestock. Foie gras (fatty duck liver) is made by feeding ducks sugary grains. It should be no surprise that kids today are suffering from nonalcoholic fatty liver disease when they stuff their faces with sugary grains every morning & some sort of bred or pasta at every other meal.

Plants protect themselves by producing toxins in an effort to be inedible or harmful to the things that seek to consume them. This is why we generally work to soak & ferment & cook them. Animals defend themselves with horns & hooves & teeth, get past those & you have food.