Time Zones factor into that. They want to be able to operate 24/7 "follow the sun"-style, so they want to get people half way around the globe - who obviously can't come to the office.
Few companies these days actually offer full remote work in their own time zone - the allure of being able to "control" your employees by making them come into the office is just too strong and "productivity" is not something that can be easily measured, let alone discerned (not by the people who make the decisions, anyway).