Why is this argument rejected by most economist?

Because cheap foreign labor undercuts the labor force already in the US. Instead of paying someone $10.00 an hour, the employer finds someone to work for $5.00 an hour. Cheap foreign labor is generally here illegally, so the employer is not paying into workers compensation, unemployment insurance, health insurance or any other benefit. It also results in a lower standard of living.

That is what was explained to me by my former econ teacher. Take it for what it is worth.