I don't even know if this is arguable. Most service jobs likely could be trained within a week at most. The most complicated part generally involves learning how to operate a point of sale system. Most of them could be operated by anyone, which is why self checkouts are a thing now.
Farming on the other hand, especially modern farming, must take a lot longer to learn. There is a lot more to learn about your equipment alone, nevermind your crops, fertilizers, rotating fields, weather patterns, etc.
