Illegal immigration and the Information Age

We are told by experts that the U.S. is moving from the Industrial Age to the Information Age. Old manual labor jobs are leaving and are being replaced by "knowledge worker" jobs.

We are told by political and business leaders that the U.S. "needs" to import manual laborers from less-developed countries, and provide amnesty for previously-imported illegal workers (largely employed on farms and in factories).

Can someone please explain how it is that, when the number of manual jobs is going down in the United States, we need more manual laborers?