I can't vouch for your situation, but my parents let their almonds stay on the tree this year until someone offered to harvest them in exchange for a good chunk of the produce, and last year they ended up using the almonds they harvested for Winter fuel.
It costs money to harvest crops, more money to prep them for sale and more money to till the land over again afterwards. Last time my parents harvested almonds for sale, they were offered all told less than €100 for their entire crop, covering 2 weeks of back-breaking manual harvesting and shelling labour in addition to the ongoing tending of the trees. The offer would have had them running at a significant loss - it was financially better for them to burn the nuts as fuel and save money on firewood.
If buyers are pinching hard enough, it might get to the point where harvesting crops for sale could cost the farmer money.
On the other hand, I know some crops can be dried on the stem. When I'm growing chickpeas, garden peas and beans, I sometimes let the plants die off with the peas inside and shell them, ready dried, for Winter storage. Could some of those fields be for popping corn or other dried corn?