If you're Bitnami it probably made sense to do it the image the way they did, but for everyone else, it's just a massive complication.
Personally I don't understand why anyone would have opted to use the Bitnami images for most things. They are really large and complex images and in most cases you'd probably be better of building your own images instead.
My guess is that there's a very small overlap between people who want to maintain Docker images, and the people who chose to run Bitnamis images.