Agri-business depends on low wage workers and has done throughout US history. (Think slaves and indentured servants.) The service and construction industries are now also dependent on immigrant workers. Disappearing undocumented workers would have a huge impact on business as usual. So the owning class likes it this way, reaping huge profits off the backs of the poor.
The industrial-private-prison complex sees the undocumented as cash cows. Detain (incarcerate), deport, etc with a profit to be made on each body. Once again, the owning class likes it this way.
The last thing the Koch's and ALEC want is a sensible immigration policy. Passing free trade deals and other neoliberal economic and military policies that destroy Mexican and other developing economies drive immigrants out of their homes, off their ancestral lands, across the US border, and into the shadows where abuse and crimes against them can easily be hidden. SOP.
Laws are passed that criminalize their existence or make it damn hard to survive, like legally driving to their low-paying job as a farmworker where they are underpaid, overworked, exposed to pesticides and the weather all day long, to put food on our tables.
US policies force them here, force them into shadows and strip them of human rights. The owning class keeps on criminalizing more and more behavior, and the private jails (detention centers) being built at breakneck speed (ironically with the labor of undocumented or low wage immigrant labor) remain full.
It should be noted that immigrants in every category and demographic commit less crime than native born Americans: we cannot count on them to murder, rape, and steal their way into our jails. Criminalizing minor infractions of the law (crossing the border, driving without a license) is the best way to ensure the profits of the private incarceration industry and agribusiness going forward.
We really need a Justice system. Poder para el pueblo!