[danaris]

The proximate reason can be "because we've always done it this way," or "because it's easy", but I'm not sure that changes the root cause.

Ultimately, it comes down to "this is something that could be done better by a trained adult, but we don't have the same access to trained adults that we do to children, in part because of the massive economic inequalities our society experiences today."

[Retric]

No, the proximate cause is earths rotation and thus plants are vastly less flexible than factories.

I need someone for to do a seasonal jobs for a few weeks and then nothing until next year is fine if it’s a niche job like say selling Christmas trees. However, when every farm for thousands of miles all needs that same labor for those same short window you get a massive labor shortage. Worse modern farming is mechanized so you can simply put people picked off the streets behind the wheels of 100’s of thousands of dollar pieces of equipment.

There’s a similar exception in US immigration for migrant foreign workers to pick fruits and vegetables, but again the minimum threshold for competence in other types of farm labor is higher.