Why can you only get a dragon on oblivion on PC and not no 360

Obtaining a dragon is only viable in the PC version of The Elder Scrolls IV: Oblivion because one person in the modding community created it him/herself and decided to share it with the public. The mods are made for the PC.

Other options are available to see one of those legendary beasts. A monument of a dragon can be seen in the Imperial City. During the final quest of the main story, one of the Nine Divines, Akatosh, will be there. Akatosh's form is that of a dragon, the same one from that statue found in Imperial City. One of the playable races, Argonians, are humanoid dragons.