Because western civilization developed differently, Westerners have different customs and traditions. You ask why, but the answers are in the history of Western civilization. I'm not going to dig up a time line and explain the evolution of European philosophy, art, and custom. You can use the Internet and books, you know.
http://en.wikipedia.org/wiki/Kiss#Kissing_in_Western_culture
http://en.wikipedia.org/wiki/Kiss#Kissing_in_Western_culture