Why do the top 10 heroes in Boku no Hero Academia only come from Japan? In Boku no Hero Academia, we know the world is not limited to only Japan; there are America and other countries as stated especially in the BNHA: Two Heroes movie. But why are the known top 10 heroes (probably all the best heroes) Japanese? Endeavor, Jeanist, Edgeshot, etc. are all from Japan, even All Might (although he's been in the USA doing hero work).

Is there another top hero that is actually not Japanese or not from Japan? Or is it just that Kohei Horikoshi makes Japan so much stronger in this series?
The Hero Billboard Chart that is usually mentioned in the manga is not the world rankings. This should be obvious since only Japanese heroes are in there and since it is named Hero Billboard Chart JP in the manga. 

To quote from the wiki,


  The Hero Billboard Chart JP is an event that presents the official rankings of Japanese Pro Heroes.


If I have to guess, no top heroes from other nations are mentioned since mentioning them would not be relevant to the story as it is for now.