It's a cost issue usually disguised as a "skilled immigrant" issue. Large companies want technical workers (the new miners and factory workers) who are willing to accept a much lower wage than native workers. So when they talk about "skilled" they usually mean "skilled and willing to work for $40k/year instead of $80k/year".
Just look at how many H1B positions are advertised and how much legal but shady stuff goes on. Put ad in local paper among classifieds for "free dirt available", if any resumes are received from US citizens they are discarded (note they have to be "considered" but not accepted), and so on.
If it was just about "skills" there would be no need for this shit to go on.