There is an issue with 'diversity in tech' and, among other things, under-representation of African-Americans in STEM.

So why are the big tech corporations that are constantly virtue signalling about Black Lives Matter and social justice not simply training American engineers (not specifically black, but people coming from poor socioeconomic backgrounds, which would definitely include many African-Americans). Why do they bring Indian or Chinese engineers?

America does not lack people, it has good universities. It could certainly train a local workforce. Providing good jobs to those communities would do wonders to reduce criminality, drug abuse and various issues.