Because the demand for good workers doesn't go away when the supply of good workers gets decreased. To be clear, I am supportive of immigrants working whatever jobs they want to work and that people will hire them for (and I'd argue that the free market is also supportive of this), I'm just noting that people have a financial motivation to keep people more competent than them out of the labor pool.

(Unless, of course, the replacement of competent workers with incompetent ones causes the economy to shrink, which now that I think about it is pretty likely.)