Well, that's a question of payment. Raise the salaries, and Americans will flock to those sectors. Instead of artificially keeping the margins up and the salaries low through labor import (which would end to the lowest common denominator race to the bottom among world economies, but usually it stops midway just lowering the options for the higher economies).

That argument is akin to the argument that we need slaves. Illegal immigrants are second class citizens, they don't enjoy nearly as many rights that legal immigrants does. They can be paid less than minimum wage, be worked harder than legally allowed etc. Of course the low skill sector loves having them, who wouldn't want to hire slaves who can't fight back when you abuse them?

But I argue that California have no need for slaves. If a job can't be filled with legal workers then you raise the wages and conditions. If you can't afford that then the job obviously wasn't critical to our economy and we should let it disappear.