Here's why:
* Xbox 360 Arcade lacked a hard drive: The Xbox 360 Arcade model came without a hard drive, which was necessary to play original Xbox games.
* Xbox 360 Arcade wasn't backwards compatible: Unlike the later Xbox 360 models, the Arcade version wasn't backwards compatible with original Xbox games.
To play Halo and Halo 2 on an Xbox 360, you would need:
* An Xbox 360 model with a hard drive: The Elite and later models had built-in hard drives.
* A hard drive to install original Xbox games: You could install original Xbox games on a hard drive connected to a non-Elite Xbox 360.
* The games: You'd need physical copies of Halo and Halo 2 for the original Xbox.
However, you can play Halo 3, Halo 3: ODST, and Halo Reach on the Xbox 360 Arcade. These games were released specifically for the Xbox 360.