The multinational corporations (Nike, Reebok, Monsanto, ConAgra, Levi Strauss, don't want to pay anyone a living wage so they will export as many jobs as they can to the place they find the cheapest labor, and where they do not have to pay any attention to workers rights. If they cannot export the job they lure illegal immigrants in to do the slave work for them. Anyone who thinks that the entire 12 million+ sneaked themselves over the border without some assistance is kidding themselves. And no it's not just the relatives sneaking them over here. If the multinational companies do it then the smaller businesses are then forced to take up similar practices in order to stay in business.
No, it's not that American's won't do these jobs. It's that American's cannot afford to do these jobs at the wages and conditions set down by the employers. The illegals will work for less and in sub-standard conditions because they live under the constant threat of being sent back home if they complain too much. It's much harder to bully American workers like that.
Books like Eric Schlosser's book "Fast Food Nation" tried to expose these facts, but as usual American's crowing about immigrants either didn't read these books, simply don't care, or are part of the United States of Amnesia.