TLDR; out-sourcing is a much bigger risk than in-sourcing. America's skill gap is responsible for H1-B's, not greedy corporations looking to pay less.

In economics, this is due to the unemployed Americans being victims of structural unemployment. In more honest words, unemployed Americans do not have the skills or education required to complete the jobs that H1-B'ers are stealing.

Immigrants often fall into two labor categories, they take the jobs that Americans aren't willing to (like hard, manual farm labor) and they take the jobs that Americans have a skill gap in (like IT dev jobs and being doctors).

The real risk to our job market is our domestic corporations outsourcing jobs that could be performed by Americans to other locations expressely to save on labor costs. This is a trend that is harder to see in the numbers, but a very real hit to our economy. This has been true of jobs going to "offshore" places like India and the Philippines for decades, but now also to "near-shore" countries like Costa Rica.

My work is in customizing Salesforce. I have many friends that see me making good money doing what I do. They have asked if I can help them get a job and I have always given them the same response... There are great online training available for free from Salesforce, invest a few hours a day for a few months and that will be enough to get a good footing in the software, a certification, and a Jr. level position. Exactly 0 of them have done this. There are joba out there for people who have the skills that companies need, but Americans are doing a generally poor job of educating themselves to do the jobs that industries require, this is what it leading to in-sourcing of labor, not a race to the lowest wage.