It is the value, ie. money, generated somewhere are coming to America as an investment. Humongous amount of money in the world is trying to find any investment worth investing. Think why a Russian investor invests in San Francisco Silicon Valley instead of, for example, Russian government promoted "Russian Silicon Valley" :)
"The large returns generated by start-ups "
Don't worry - these large returns, in the form of high paying jobs, technologies, experienced teams, and large share of profits reinvested - the things all other countries are scramble for - it will be left here, in the Silicon Valley.