The point of the shichibukai is to bring balance to the world. There's the yonkou who are the top pirates then there's the orginization known as the world government. The shichibukai are pirates/government to ensure the balance doesn't get too unstable on a side so it would overtake the other. Since we're missing a yonkou and more than half the shichibukai, that's why the oceans are in chaos now since the balance is currently unstable