…Why are humans considered to be a mess if cultures? By who? Compared to who? For all we know we might be in a galaxy full of diverse cultures and actually be considered exceptionally plain and boring in comparison with any one of them. …
Because SciFi is usually used as a "mirror" for humanity, as a way to write/talk/show about very human topics. Aliens are then used to showcase a specific aspect/idea/concept of human nature the author wants to talk about.
The result are aliens that are very homogenous*
Outside the context of SciFi, members of an alien species would probably look extremely similar - if not identical - to humans. How many humans can differentiate between different individuals of chimpanzees, bonobos, dolphins, robins or lions?
If an alien species is very varied in form and culture humans would probably simply assume that these are different species.
In short, the problem would probably be the inability of humans to differentiate between individuals of a completely different species.
*) and boring as often the intention of the author is obvious
…The topic of this particular thread is the Guardian SLFs and FD have established lore covering why things are as they are in this particular context.
I'm searching for that lore explanation from FDev. Could you point me to it as I can't find it. Thanks.