And to add on to what Phoenix just said, the only reason that the plot seems to be focused on Sephiroth is that the point of view is pretty much only seen through the eyes of Cloud and his party. Cloud is so blindly chasing Sephiroth that he and the rest of the party kind of overlook everything going on around them. So unless the player of the game is looking to read inbetween the lines of the story and play close attention to detail, it will easily seem that it is nothing but a chase for Sephiroth.