Why do jedi disappear when they die?

Answer 1

Jedi Masters disappears when they die because of a techniques they learn which gives their concisciences form after death. But not all Jedis learned this technique because in Star Wars: Episode II - Attack of the Clones the Jedi masters that died did not disappear- Obi-Wan Kenobi and Yoda taught themselves- Yoda told obi at the end of Star Wars: Episode III - Revenge of the Sith that he had training for him- thus he taught him to do it

Answer 2

In the Star Wars universe, Jedi are able to achieve immortality through the Force. When a Jedi dies, their physical body fades away as they become one with the Force, preserving their consciousness and allowing them to communicate with living Jedi through the Force. This ability to become "Force ghosts" is a powerful manifestation of their connection to the Force.