Why did the Tet offensive shock Americans?

I personally remember that period of history, and I don't think that America was all that shocked; Americans knew that there was a war taking place in Vietnam, and during times of war, it is understandable that there will sometimes be an offensive. However, to some extent, Americans had become used to winning wars, after essentially an unbroken string of victory, going back to the Revolutionary War. Nobody ever defeated America. That being the case, it was easy to believe that the North Vietnamese should simply admit that they can't win, and should give up. Instead, they fought with ceaseless determination. It seemed, if not shocking, at least odd that the Vietcong thought they could do what Kaiser Wilhelm, Adolf Hitler, and Kim Il Sung had all failed to do. What made them so special? Yet, they did succeed, eventually, and the US withdrew from Vietnam in 1975, an unprecedented event.