Agreed, I've spent quite a bit of time in Bangkok and Tokyo and while the Thai and Japanese food is incredible in those two cities, the selection of other ethnic foods is pretty limited. You won't for example find Mexican, Brazilian or Moroccan food in these cities. When it comes to both quality and selection of ethnic food from around the world it's hard to beat the large US cities.
Speaking as a Tokyoite, you absolutely can but you just won't see them if you're a tourist staying for a week in the most central parts of the city. All in all, it's true that there's more variety in US cities - but I guess I don't value variety as much as you do.
