You sound naive to me.
Roofing work is generally hot, dirty, with long hours, and physically demanding. There are not enough native born crews to handle all the work that is out there.
Half the roofers in the US are foreign born. About a third are undocumented.
To assume that they are doing the work that´s out there because they can be paid less than US born workers is naive.
Reporting Finds Roofing Employs 75,000 Undocumented Workers
Reporting shows that undocumented workers make up 10% of U.S. construction labor, with 75,600 roofers among them, highlighting the industry's heavy reliance on migrant labor to fill critical job shortages.
The University of Michigan’s case study concluded the usually unspoken truism: subcontractors favor hiring undocumented workers because it saves them money and allows them to produce more competitive bids. It is estimated that U.S.-born construction workers earn an average of $3.12 more per hour than undocumented workers.