It's because the idea of continents was started in antiquity and back then the idea of continents was based on ethnic and cultural difference and not any real geological reason. Once you crossed the caucuses you started encountering "Asian peoples". Go south to Africa and find "black peoples". In fact it used to be just east-west continents in earliest times and the Euro's considered Africa on an east and west divide too.
So continents as being the largest land divide as invented by the Greeks and other early predecessors to modern culture didn't create those divides and name them based on geography or geology, but on what boundries one starts to encounter different "races".
