That division line was drawn well before the Americas were discovered, when the civilizations we decended from thought the world consisted of Europe, Asia, and Africa; Europe being west of Asia.
Because most of the developed countries are geographically located west of most of the emerging developing nation unless you live in Central/ South America then its different
It might be from a soviet-era stand point (west of the iron curtain.) And particularly the U.S. is geographically in the western hemisphere and referred to as a 'western country'.