I have a feeling that part of the answer to this question is that schools in China, India, Russia, and Ukraine (that you did not mention) are very good in terms of mathematics and computer science. So applicants from the above countries are very competitive. And I can add that, in general, an average US student does not know as much math as students from China, India, Russia, and Ukraine. For example, Calculus is taught to 15-16 year olds in high schools in Russia, but it is usually a 1st year math class in the US. The same applies to other sciences like Physics and Chemistry.
In addition, what other professions do we have in the US? We need doctors, lawyers, business owners, politicians, and other people with non-quantitative skills. To get into these, you need strong verbal skills which not many applicants from China, India, Russia, and Ukraine have. So they go to study quantitative subjects like math, statistics, computer science, engineering etc.