Hasn’t been that way for a long time, though it has gotten worse in the last 20 years.
My grandpa planted oats and wheat on one field, while having a rotation of alfalfa to “enrich” the soil (good nitrogen fixer).
Some farmers do that. Now the focus is on corn, using fertilizer to regenerate the soil. Some still use corn/beans/alfalfa rotation, but they are in the minority.
corn, wheat, beans were always the staples in our area of Ohio. If the field wasn’t in one, it was in the other. Most planted some for rotational reasons, but primarily for protection against changing markets. Many also did all 3 and fed out beef/hogs to provide even more diversity.
They tended to stay away from dairy. Too work intensive and steals the hours of the day and the days of a life.