The US needs a cheap labor source for largely our agricultural industry. They were used also for the construction boom(I saw literally armies of Mexicans in the Ft. Myers/Naples area during the housing boom). Democrats see a potential huge voting bloc that would virtually assure them permanent majorities in the Senate and House as well as the Presidency. Republicans are no better in this regard when it comes to illegal immigration. The Central Americans pouring into our borders also help support our huge criminal "justice" industry......private prisons, attorneys, militarized police, etc...Keeping those jails and prisons packed.
Chinese slave-labor produced goods now enter Mexican ports rather than US parts in order to bypass the Unionized Longshoremen.....Fleets of Mexican trucks laden with cheap garbage driven by under-trained and underpaid Mexican drivers share our roads to dump their loads into soulless gray big box stores so hordes of fat overweight Americans can consume the shit.
The US has an empire to maintain. Every great civilization had large swathes of its populations as nothing more than a slave-class or serfdom. Hell, in a few cities during the height of the Roman Empire slaves outnumbered citizens. Today, with the advent of globalization, the populations of China and the sub-contracted populations of Vietnam and Indonesia exist solely to provide Western nations with goods.
China may rattle its saber and pretend to be an emerging superpower, but with zero innovation and the fact that the US has undetectable Trident submarines lurking off its coast, the US will always remain supreme in the near foreseeable future.