I've never bought the idea that we couldn't find American workers for these jobs; but in the great economy in the 1990-2000's other jobs were available at higher pay, so many
Americans rejected these jobs.
One of the minor reasons for the housing bubble was the availability of cheap labor. If the builders had to pay higher wages to attract legal workers it would of driven up costs; but it would of slowed down the final bubble popping. Since the mortgages were so expensive, the increase price for labor would have easily fit in the monthly payment and would not have been too much of a brake on sales.