As an immigrant, I believe that US education is the number one cause for most of this. I did my undergraduate degree from Nepal and came to US to go to Grad school. Most of the engineering course content is American and most of us end up being inspired by the pioneers - Dennis Ritchie, Linus, Bill Gates, John Carmack to name a few

Once you are here, and have a degree, there are opportunities here that are not there back home and hence the interest in working visas.

Consultancies are another catalyst - in cases where the candidates do not manage to get a job right out of school, consultancies act as a bridge where they file immigrant visas for the candidates and then help them find companies that are in need of their skills.