I think it has to do with growing in a small community (of which being a farmer is often a direct consequence).
If you live all your life in the same environment you start to believe that it is the only right way to live, while instead if you have many different life experiences and meet people with different backgrounds you start noticing what everyone has in common and that becomes the focus of how you perceive the world.

I mean, there's way too much going in to it but the tl;Dr is because it's in their class interests as owners of significant assets (the average US farmer has $1m in land at a minimum) and the profit margins of the larger owners are often built on exploiting vulnerable labor.

That they frequently lobby against their own financial interests in every other aspect? I don't think that's something you can go into without writing a full essay.