"Why the amalgamate takes the shapes it does is up to anyone's guess, really. We cannot decide what we would want to look like, as what we can look like is quite well defined in our genetics, granted we can change bits and pieces here and there, but largely you are what you were born with. Perhaps this unshapen mass mimics those around it, learns from seeing and sensing other life forms and then mixing and matching what it has witnessed, ultimately unable to really form anything outside of the things it has seen. Or perhaps it has a sense of aesthetics and it finds these shapes to particularly pleasing. The former option would be quite interesting, as that would mean that there could be visual aspects of alien life forms in it, life forms which it possibly has consumed or seen in the far reaches of the universe, or wherever it came from.