Because the amount that they'd have to pay Americans to do those jobs is way more than they're willing to pay. It would drive costs up a lot. There's even been surveys when asked who would want to work in the "factories" that Trump promised and it's a very small number
https://reason.com/2025/04/15/do-americans-really-want-to-bring-back-manufacturing/
I know that's not picking crops or whatever, but it's a similar concept. If, for example, a meat plant had to hire domestic workers, they'd have to pay them quite a bit, which would drive up the price of meat by a lot. I sure as hell wouldn't want an 85% or whatever it is pay cut to work in a meat plant.
I am not saying we should have totally open borders or anything like that. That's retarded. However, an easier way to bring in immigrants legally who want to work in jobs like that would benefit everyone. You could even make it so that the factory would have incentives to hire citizens (like make them pay a tax on wages for the people brought over or something like that). And then of course you could make it so that if a company is caught hiring someone who isn't here legally, they get severely punished (funny how this doesn't happen now, lol).