The original trilogy seems to indicate that Ben was only wearing those robes because it was what you wear in the desert; Uncle Owen was too.
I know this has been widely debated if this is dumb or not (I think it is).
Sure, Anakin’s ghost is seen in robes at the end of Jedi but he did live on Tatooine, so that wouldn’t have to mean it’s the jedi outfit.

Does anyone know at what point George decided that the robes would be the Jedi uniforms? I believe there’s some early concept art for Episode I showing the Jedi wearing something resembling what Luke wears in Jedi, but correct me if I’m wrong.