Because it is not up to him if he is out or not. It's the umpire which control his faith, whether out or not. But when it comes to fielding, umpires aren't always sure about whether a ball was a wicket and how they act on the field, builds a reputation on the players, the team and Ricky Ponting. Or is there some need for players to lie about whether they caught the ball or not?
Clarke caught the ball, reaction was made, he had a general idea of what happened, looked at Bucknor, he nodded and just glanced at Ponting and was enough to say he was out.