The PER unit cost of picking fruit/veg is almost nothing.
Apparently the savings were enough to justify the cost of the machines that replaced the pickers. There might also be the time factor as well—the machine might be able to work several fields in one day that the gang of workers it replaced could do in a week. The long-term trend in agriculture going back to the 19th century has been to replace lots of workers with few machines. And we end up with tomatoes that have no flavor.