Custom OnSceneGUI-editor not working for ScriptableObjects?

Hello Internet,

I have a ScriptableObject...

 [CreateAssetMenuAttribute]
 public class PickupPosition : ScriptableObject
 {
     public Vector3 _position;
 }

I want to visually edit the _position attribute within my scene. Preferably with a transform-handle:

 [CustomEditor(typeof(PickupPosition))]
 public class AreaEditor : Editor
 {
     public override void OnInspectorGUI()
     {
         Debug.Log("The Inspector-GUI gets called");
     }

     void OnSceneGUI()
     {
         Debug.Log("why is the SceneGUI not called?");
     }
 }

Defining the OnSceneGUI works for MonoBehaviours but not for ScriptableObjects. Editing properties of ScriptableObjects within the 3d-scene would be quiet convenient, but I can't find any documentation if or how this would be possible.

On a side note: I have a difficult time figuring out why some methods (like OnInspectorGUI()) needs to be overridden whereas others ( OnSceneGUI() , OnEnable(), etc.) just need to be defined. I find this distinction quite arbitrary. Is there any documentation on that topic?

Any ideas or suggestions are highly welcome.