You can't compare value-add manufacturing to agriculture because former is demand-driven while the latter is supply-driven. If a mill has a customer, its ability to make money depends on expertise and equipment but a farmer can only sell what nature allows them to grow. Modern agriculture is a balancing act between the forces of nature and financing so if a farm's output one year is bad enough they are almost certainly doomed to bankruptcy within a few years unless they have a miraculous turnaround. The vast majority of farms take on debt every year for fertilizer, seasonal labor, maintenance, etc and if they have a bad year, the extra debt makes it much harder to make a profit the next year when they again take on even more debt for the growing season.

The rise of industrial farming can be attributed more to transportation than automation because it allowed corporations to form that could finance hundreds or thousands of farms across a region or continent. A disease or drought impacting the yield in one area no longer meant that those farms were doomed to bankruptcy because the profits from successful farms paid for natural volatility. In this sense, agricultural conglomerates are essentially insurance companies.

Automation and other technology would have to help farmers deal with the financially ruinous harvests to really help local farming become competitive.