Why Startups Condense in America

Introduction
Paul Graham is an essayist, programmer, and programming language designer. In 1995 he developed with Robert Morris the first web-based application, Viaweb, which was acquired by Yahoo in 1998. In 2002 he described a simple Bayesian spam filter that inspired most current filters. He's currently working on a new programming language called Arc.
Paul Graham has an interesting article about reasons why there are so many startups in US. The list includes items such as allowing immigration, being a rich country, not being (yet) a police state, good universities, freedom to fire people, and so on.
Startups are largely an American phenomenon. Why? What is it about America that makes startups work there? Could Silicon Valley be replicated in another country?
These valid points that differentiate USA.
The US has (ten) significant advantages:
The US allows immigration
The US is not a poor country
The US is not yet a police state
American universities are better
You can fire people
In America work is less identified with employment
America is not too fussy
America has a huge domestic market
America has venture capital
America has dynamic typing for careers