Impossible may not be the right word, but highly unlikely due to the environment ag automation needs to operate in. Production agriculture is highly mechanical, meaning things wear out and break all the time, which means humans need to go and fix things. In addition, people farm in every environment on earth, with wide variance in conditions in even the most ideal places. This means mechanical equipment needs to be specialized for the crop or location (considering humidity, temperature, wind, sun, crop type, etc), which makes scalability limited. These are the two biggest issues that tech doesn't really have a good solution for.