Because their cheap labor is the closest, and most readily available to obtain, and allows them downwardly bargain the wages of current American citizen and resident job holders. The industries gain a short term profit reducing the labor "cost", and reward and encourage lawmakers by kicking back some of the profits as campaign contributions.
Just another way to create an American underclass. Which is profitable for some, in the short term.
For the industrial worker or technical that has seen his/her job disappear..the transition to **Something is at least a possibility as they collect their lives and regroup in a new job orinetation direction..their mortgage and family bills motivation enough to get moving.
But get moving to what?....here the lower paying jobs which abound are consumed by illegals..the resident American now has stress compunding as $$$ figures mount and is set against them as per time.
How many Americans are willing to work a shit-bird job just to get things moving..get some cash and work their available time to find a better paying job or new career.
In their tormented sleep..the immigrant grins at them....sleeping pills become a new reality.