My tier 2 US tech Bay Area office is 80% immigrants from one country on visa (H1-B or OPT). They never say no, and they are willing to work any hours and take meetings at any time. 90% of management up to CTO is from the same country. This is for tech, its a little less lopsided in non-tech divisions. Most of them used this pipeline (I've checked LinkedIn): 2 years of non-spectacular experience in home country, cheap US master, possibly internship in US company with same ethnic makeup, then OPT, then shoot for the H1-B in company with manager from same country.

American grads now compete with ethnic hiring, foreign work morals, and possibly billions of people with not-so-impressive experience, which I assume is why we see the crowding out of locals. Add to that some groups generally having less scruples about cheating and lying in the application process, as well as being trained in school for passing tests rather than performance, and you get what we see now.