transliteration of our word and as a noun for the continent.
The reason is that from their point of view it makes no sense. And they all have a common word for 'westerner' that none of the 'western' languages have.
There is no word in the native 'asian' languages that would sensibly tie all of the different races/groups in that area into a single word, just as their is no commonly used word that we would use in every day language that ties in all of the people that they use in their description of 'westerner'. All 'asian languages have a common word that is native to their language for 'westerner'.
We don't normally have a word that we would naturally use to describe a Russian, Turk, Portugese, Brit, American and Brazillian, it is too diverse a group. In the same way 'asians' cannot see a why you would tie a Chinese, Japanese, Malay, Burmese all in together in the same word, it is too diverse.
All 'asian' languages have such a word;
http://en.wikipedia.org/wiki/List_of_slang_terms_for_white_people_in_non-Western_countries
Some of the more well known ones are
gweilo = Chinese
farang = Thai
gaigin or hakugin = Japanese
In English we have the archaic and academic words 'oriental' and 'occidental' which are so broad to be meaningless because the group that these words identify don't naturally occur in our minds or language, it is too diverse and meaningless.
Ironically the languages and cultures of Russia, France, Portugal, Canada, US and Mexico are much more integrated and similar (sharing culture, religion and language roots) than Vietnames, Cambodia, Thailand, Burmese, and Malaysia even though they are all in the same neighborhood.
Between Vietnamese, Cambodia, Thailand, Burmese, and Malaysia there is NO similarity between language and very very few words that are borrowed across linguistic lines. These languages share nothing in structure, tone, vocabulary, tonation or base vocabulary. There is more similarity between Russian and Portugese than any of these languages even though they are all within a few hundered miles of each other.
This shows how geography kept these cultures seperate and distinct from one another with only Buddhist and later Islamic missionaries having any cross group impact.
The mountain ranges and rivers that seperated these countries made successful military and occupation campaigns difficult so that except for occassional raids there was little military interaction between the countries. Where it was possible to have such occupation, China, a dominant central administration took root that squeezed out and dominated the approximate 290 different language groups that lived in the area;
http://en.wikipedia.org/wiki/Languages_of_China
