America is a *lot* younger than Europe or East-Asia, by the time European settlers came to the shores of the USA, or even before the continent was known to either, both had long-established artistic traditions.
American artistic traditions have always been intertwined with European art as they have been traditionally the same stock of people and in constant contact. But at the same time, American art is influenced by its surroundings, just as all art is. In Europe, nature had long been a conquered thing, but even today in America there are parts that are relatively unexplored (consider the American Northwest, Canada, Alaska); European and East Asian art often focuses on the powerful, but America was England's experiment in Liberalism, and so it focuses on ordinary men.
American art has its own character, and if allowed to continue, will eventually reach the same richness and depth as the Europeans'. I personally am a big fan of American art.