>>1. Why do we need to bring in immigrants to found companies? Do we have a shortage of Americans already here who also want to start companies?
This is the equivalent of asking why large companies acquire start ups and don't in house innovate their way out to compete in the market.
The answer is, the existing inertia is difficult to change in big companies/countries. Plus people don't realize the importance of the opportunity they have in hand. Imagine if people are that desperate to come to US to just start a company, its obvious the thing called 'citizenship of the US' is a very prized opportunity which most US citizens don't realize.
>>2. Or, is Start-up Founder one of those jobs, like picking cabbage, that Americans "just won't do"?
Starting up is difficult every where, but the US ecosystem is better in terms of the overall equation.
First world country, VC's, Infrastructure etc etc