The same people dependent on cheap migrant labor seek to criminalize it. I suppose this gives them more leverage with which to exploit. If the right were serious about ending illegal immigration it would mean going after employers, not the migrants, and if successful it would mean paying people enough that they are willing to do unpleasant jobs. They don’t want that, they want a convenient scapegoat with which to fearmonger and labor they can exploit. As noncitizens, this group doesn’t have political power to push back, and they can be made to seem scary and foreign to their xenophobic base.