Why do I have to replay the last level of Halo 3 each time I start the game, even though I have finished it?

I would like to go back and play the levels of Halo 3 that I enjoyed most (like I did with Halo and Halo 2) after completing the game (solo, normal). But for some reason, I always have to replay the last level driving over the crumbling terrain and jump into the ship. No matter what I do at this point, every time I start the game, I can only choose this level - as if I haven't completed the game. What am I doing wrong?