Hi there,
Obj files you import doesn't generate materials, you must assign a new material in the inspector, but this new material is never visible in the resources folders ?
While other formats sometimes create material previews

Why can't we create material files directly under the resources folders ?
Why there is no Right click on menu and choose Create new material ?
Or sometimes you want to drag and drop a material on another object, it's more easy when the material file preview is visible in the resources folder.
I find it counter intuitive to not have a preview of all your materials; or quickly find a material preview than duplicate it before changing some textures and and drag and drop it on another model.