Google is of a size that requires distributed teams. It's hard to hire that many top engineers in one place, particularly given their laser focus on the top of the talent distribution. They pay a performance tax for having to distribute, but they get to marshall engineering resources most firms only dream of. And their culture is very consistent.

Did you notice that they were developed most powerful code-reviewing tools? That is an evolution of the pair-programming concept from an ortodox XP. ^_^