Yeah. I think what it is, is they're not attracted to doing farm work, farm work for low wages. Some don't want to work but most don't want to do that kind of work. If you have an immigrant who's used to making $10 to $30 dollars a day in his/her country (and that's being generous) then it's no wonder they don't mind doing the farm work here for $3 to $8 per hour or being paid by the amount you pick. Most Americans don't and won't do that type of work for that amount of money. But while it's not a lot of money for the immigrant it's more than they may have made back home and generally more than one family member is working which increases the take home pay.
I can see your point, and this is one reason why many support a guest worker program where workers in this category can cross over legally, work seasonal employment, then return home. But for those who employ them, they no longer would enjoy the benefits that come from having an "underground" workforce. Let's face it, a lot of these employers are simply exploiters, taking advantage of workers who are desperate for work and are vulnerable due to their immigration status. The activities of the "coyotes" who bring them over are absolutely reprehensible, keeping them virtual prisoners and slaves until they pay off the costs of being escorted over the border. They often live in grisly conditions.
Sure, they may make more money, but that's only because the conditions back home are so dreadful and with very few opportunities for advancement. NAFTA was supposed to help fix problems like that, but it never did.
I think the obvious answer is population growth and demand. In order to keep up with that demand or even quotas they have to hire people to do the work and since it's not something most Americans want to do it's a niche for immigrants to fill.
I'm still not entirely convinced that it's the actual "work" that Americans don't want to do. It's the pay and the working conditions. Sure, it might mean cheaper food prices, but in the long run, I'm not sure that it's really worth it. Back when cotton plantation owners needed to import labor, they sold America's soul in the process. They made many of the same claims about "lazy" workers, but I never thought it was true.