You're forgetting the nuance that all of this is strictly regulated by the rules. You can't just go and challenge the clan leader to a duel. You have to do it at a strictly defined time and only after fulfilling a number of requirements. And only then, in order to become the leader yourself. And the whole point of the ritual is to make the clan stronger. If you just kill the leader and ride off into the sunset, you're not making the clan stronger. Quite the opposite.
Although, maybe they just found out he's from Vesteria...
Where do we end up when we save the world?