Because many of Trump's base are low-educated rural people who see the American jobs being generated by the best foreign entrepreneurs as requiring higher education and skills than they possess.
I'm a high-income, highly educated immigrant. I grew up among similar people, and witnessed the extreme derision directed at "low-educated rural people." Given that, it's a bit difficult for me to blame Trump's base for not wanting more of us here.
Eh, I don't blame them, but I also don't blame the high-income highly-educated immigrants for feeling derisive toward people whose only asset is that they're American. I think Tom Lehrer's 1967 observations are still remarkably on-point:
I think it's pretty hypocritical. India and China certainly don't subscribe to the "credo nation" concept that American elites have embraced. In Bangladesh, when we speak about other Bangladeshis we identify precisely where in the country their families are from. (It'd be like making it a point to note that someone was from Texas or Virginia.) But when we speak of people from outside the country, we call everyone "bideshi" ("foreigner"), regardless of nationality. An American could move to Bangladesh as a young child and live there her whole life, but she'd always be "bideshi."
This is a bit of a myth. Trump's core are older suburban conservatives who typically make the median household income or greater. Nate Silver at 538 has done a good job of breaking this down.
And while it's true that the white rural poor vote Republican, as a political force they aren't that potent. It's unfortunately, because their interests often get steamrolled by more corporate-oriented Republicans. Democrats throw their arms up and say "Stop voting against your interests!" but neither party has any coherent vision for rural America. Republicans see them purely as culture voters, and Democrats are focused on giving them the same complicated means-tested benefits as everyone else.
This is so untrue. There are many many American founders who are dying for mentorship and support and anything from VC, and many many more people who want to develop their careers in innovative ways who have degrees in STEM. Domestic innovation even if it's copy cat is socially better than importing non natives.