Part of the problem is you don't know ahead of time (certainly not with 100% certainty) who's going to be an exceptional unicorn wrangler, and who's just going to be a pretty good engineer, unless they already have an incredible track record elsewhere. This will filter out a lot of possible future unicorn wranglers.

Geez, maybe there shouldn’t be tens of thousands of hires over Americans then?

Sorry, it’s just that maybe a LOT of you aren’t understanding the motivation here?