One of the most important sectors in the American Economy is Agriculture. When it is time to harvest the land, farmers still need to pick up the crops. Sure, where it is possible mechanization is employed, but mostly it is done by hand. Harvesting is hard work, and to the average American workers, it is more profitable to do any other type of work. For this reason, farmers most often have no other choice but to employ undocumented immigrants. When enough people benefit, no one wants to change the current situation: farmers handover low wages and get higher profit; customers get food at low prices.
Food rotting in the fields occurs when the government steps in and enforces the law: forcing farmers to obtain proof of lawful status from their workers. Not enough legal workers to pick crop, crop rots. A farmer in one of the states that cracked down on illegal immigrates a few years ago said, "You want to fix illegal immigration? Stop eating."
Like I said before, the Wall is but a superficial solution to a profound problem. Illegal immigration affects all corners of the economy, and its flow is fueled by economic, narcotic, geopolitical forces.