They aren't. The first Halo was outstanding and actually provided some innovation to the genre. Halo 2 provided us with the premier console multiplayer experience, and 3 was a culmination of the two. It's a fantastic series we were lucky to have.
My Games of 2011:
The Legend of Zelda: Skyward Sword
Elder Scrolls V: Skyrim
Super Mario 3D Land
Uncharted 3: Drake's Deception