Why don't GridPane, HBox, BorderPane, VBox, StackPane have Skins?

[Alex Sviridov]

I am studying JavaFX API and can't understand why GridPane, HBox, BorderPane, VBox, StackPane don't have Skins. According to Skin API doc:
>Base class for defining the visual representation of user interface controls by defining a scene graph of nodes to represent the skin. A user interface control is abstracted behind the Skinnable interface.
Why do JavaFX developers use skins only for controls, but not for the above panes? I am asking, because these panes also have visual representation. For example, we can set color, they have children which can be placed somehow differently etc. Could anyone explain?
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Alex Sviridov