It's part of the culture, as it is in Japan. It's just an expression of courtesy.
Why do many Koreans choose to bow rather than shake hands?
Traditions in many Eastern societies hold that gratuitous physical contact, particularly in public or between strangers is considered disrespectful (or worse, dishonorable), presumptuous or uncivilized. Some will also cite a basis in hygiene.
Among the more laudable wisdoms of those cultures, IMO.