Many people are capable of doing other work but not capable of doing agricultural work; especially if we’re talking about hand field work.
Such work requires good physical condition. Doing it at any reasonable rate of speed requires physical condition that’s trained for the particular work. And doing it without screwing up requires knowing how to do the specific job – can you tell when a melon’s ripe enough to finish ripening off the vine, but not ripe enough to rot in the bin and screw up all the others? did you know that how to tell varies with the specific variety of melon, and the tell for one kind can be worse than useless for another? And that’s one specific job and one specific crop – and some of those tells are body knowledge, such as how much pressure it takes on the side of the stem, that can only be learned with practice. Not to mention that a bad melon can ruin others in the bin, but ignorance of safety practices can ruin or end a life.
Yes, most people in good physical shape can learn how; but very few can learn the skills and get into the condition fast. Yes, school kids used to do some of it – but school kids used to learn how.