Many skilled positions have scarcity, and employers are willing to pay more for an immigrant if their skillset is a better fit and/or can fill the position immediately.
>Most blue collar labor is fungible - meaning that just about any person who is not incapacitated can do it.
I think you underestimate the skill and training required for many blue-collar professions.
I probably have apprentice-level skills as a plumber, an electrician and as a mechanic. I know enough to say with some authority that it would take years for me to get to journeyman level. I don't think the average person could be a really good programmer, and I don't think the average person could be a really good master mechanic, either.
Many skilled positions have scarcity, and employers are willing to pay more for an immigrant if their skillset is a better fit and/or can fill the position immediately.