Primarily because not _every_ worker will be moved when a company moves. A lot of the workforce will be local, hence the language requirements.
The problem is that if you want to attract non-local talent, it's harder to attract them somewhere where they'll have trouble with day-to-day life. English-speaking countries have a big advantage here: no matter what nationality a professional has they probably speak English well enough to be comfortable living there.
As another poster mentioned there are cities where the overall English proficiency is so high that it matters less (Stockholm, Berlin, Amsterdam, ...)