Verified answer

because there's a ton of people that speak other languages that wouldn't like that. also, because making it the 'official' language kinda promotes both laziness and segregation... laziness because people won't bother to learn another language (even though the world is in serious need for multi-lingual people) and segregation because it will separate those who can speak the official language from those who cannot.

Because most of the world does not want to do this. Can you imagine what, like, French people would do if they tried that?