Why it is impossible to create an asset in "StreamingAsset" folder?

I'm trying to create an asset of a ScriptableObject in the StreamingAssets folder, but the result is a "blank", not editable, asset file. anything other path works fine, but in that folder it not works. I use this code:

 [MenuItem("Test/Make Some Data")]
             private static void CreateAsset()
             {
                 MyScriptableObject phrasesList = CreateInstance<MyScriptableObject>();
                 AssetDatabase.CreateAsset(phrasesList, "Assets/StreamingAssets/MyAsset.asset");
                 AssetDatabase.SaveAssets();
                 AssetDatabase.Refresh();
             }

and not work. this, instead:

 [MenuItem("Test/Make Some Data")]
         private static void CreateAsset()
         {
             MyScriptableObject phrasesList = CreateInstance<MyScriptableObject>();
             AssetDatabase.CreateAsset(phrasesList, "Assets/foobarAssets/MyAsset.asset");
             AssetDatabase.SaveAssets();
             AssetDatabase.Refresh();
         }

Works fine. There is a reason for this issue?