As for why it's not happening today, with all the news about that one little vial of cesium that's gone missing lately... I can't imagine a nuclear regulatory agency being comfortable with letting every single farm receive shipments of radioactive material.
Not until we have some sort of GPS+Satcom method of tracking these vials point-to-point.
https://en.m.wikipedia.org/wiki/Food_irradiation - "As for transportation of the radiation source, cobalt-60 is transported in special trucks that prevent release of radiation and meet standards mentioned in the Regulations for Safe Transport of Radioactive Materials of the International Atomic Energy Act."