The Bulk of Agricultural Land is Currently Devoted to Not-So-Healthy Crops

Grain is a huge fraction of US agriculture by land area. In general, grain has quite a high insulin index and so tends to lead to obesity in those who are genetically vulnerable—which is most people. See “Obesity Is Always and Everywhere an Insulin Phenomenon” and “Forget Calorie Counting; It's the Insulin Index, Stupid.” Oil is another big use for agricultural land.

Notes:

Oats have one of the lower insulin indexes among grains.
Among oils, sunflower oil, canola oil (=rapeseed oil) and peanut oil have a higher ratio of monounsaturated to polyunsaturated fat than soybean oil or corn oil. (See the table here.) There is a greater consensus for monounsaturated fat being healthy than for polyunsaturated fat being healthy.
It is important to have young children eat peanut based foods as soon as they are eating solid food in order to help avoid peanut allergies.
See more links on diet and health in “Miles Kimball on Diet and Health: A Reader's Guide.”