Because the opportunity cost equation is different for them. $10/hr working hard labor vs. easy labor is the choice for American workers; $10/hr in the U.S. versus $4/hr elsewhere is the equation for the foreign workers. On top of that, they have the option of taking the $10/hr work for 4-6 months, and then travelling back to their home country where their cost of living is much lower and it works out the be the equivalent of $25-30/hr in purchasing power.