You can't wrap your head around it because you simply don't want to. Of course I didn't mention every single potential crop. I mentioned the three most widely grown, around the entire world. Corn, rice, and soy. Yes, others would do well, but building above these crops would never work on large agricultural areas. Why? Because you need machinery to harvest large grow ops before they spoil. Farmers would never afford the human labor required to match. It will work great on smaller scale farms, people using upwards of 25 acres. What does that achieve power wise though by comparison? Not enough power.

Pastures are an issue for two reasons. One, grass needs direct sunlight to properly grow. Two, animal agriculture is a major cause of carbon emissions. We need less pastureland, and covering it doesn't help. You could convert existing pastureland into a reactor site, saving existing nature from development.

You would still need to develop new land for larger arrays. Land use that could be minimized by maximizing the possible power output.