What good is a billion dollar startup if it pays little US tax, and only employs one or two hundred high paying jobs? A 100 Million dollar company typically employs just as many high paying jobs and may pay more US tax than a lot of these tech companies.
Also, a lot of these tech companies that are "shaking up" industries are profitable at the expense of other more traditional businesses (For example: AirBnB taking business from Hotels, Uber from taxi services, etc).
So bringing in more Billion dollar unicorn startups may not be the best thing for a country.