1. They're really cheap
Immigrants often have lower standards of living compared to the people in united states, which save a lot of money for the company.
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2. They could work in long hours
Since they've come a thousand miles away into united states, they have a certain drive to burn themselves up in works to sustain their living
3. If they're illegal, they have no legal protection
Which means companies do not need to give them any healthcare or pay attention to their worker's right.