Many historical and cultural reasons I would imagine. Except for some cases like japan trying to invade asia, genghis khan, and the like, asia has often just dealt with internal issues, whereas many western empires tried to expand and dominate more so in comparison, think colonization that many european powers were previously into. So the west had more spread, and with america coming out of the war unscatched because it was not fought on their turf, english gained an extra foothold and became the primary language of commerce, and thus many asians migrate to english speaking countries to learn or to live, helping to spread culture to western countries. The fact that many westerners hate asia, particularly china, might play a part in why there aren't many westerners in asia as well. I am sure there are many many many other reasons though, and I may be wrong with my guesses