Volume node is a thin wrapping over vtkImageData. Volume node stores an image and image origin, spacing, and axis directions (because VTK did not support image direction matrix until 1-2 years ago).
Standard numpy arrays don’t store image origin, spacing, and axis directions. If you need a numpy array that can store image metadata then you can have a look at xarray. Which is supported by a number of medical image computing packages, such as ITK.
@dgobbi have you looked at xarray and how ITK interfaces with it? It could be nice if the same Python image object could be used by ITK and VTK.