The US can use a lot more legal immigration. There's nothing wrong with a skill-based immigration program that doesn't necessarily only select for Nobel prize winners. With another hundred million people, and improved land use, we're talking perhaps another 100 years of Pax Americana. I think American should do this for her own sake and for the sake of humanity.
However, there is a group that benefits from undocumented workers taking underpaid, off-the-books jobs, and they hold significant influence in policymaking.