We refer to beings from Betazed as betazoids, from Vulcan as vulcans, from Romulus as romulans, etc... Why are we referred to consistently as humans and not earthlings or earthans?
We don't refer to the Klingons as "Kronos-eons" or "Kronoslings" either. So I guess it is like not refering to the Dutch as "Netherlanders" - the name of the race was chosen independently of the name of the planet (country) they came from.
It is probably also to distinguish us from the other sentient inhabitants:
- Voth and
- Humpback whales
The Universal Translator is really the root cause of this confusion. The languages for these various cultures may simply lack a specific secondary word to describe beings from that culture. For example, we Humans will describe ourselves as belonging to a planet or country. (Earthling, Martian, so on.) It is conceivable that not all alien cultures will follow the same structure.
A culture that lacks gods, for example, may lack a word to distinguish between those gods and 'normal' people. Or, a culture with particularly cruel gods might deem themselves unworthy of a collective title. Still other cultures may not hold tradition with as much value and have self-selected a title that is easier to remember and which requires no knowledge of their history to understand.
Cultures that lack this noun, would lack any species designation beyond a scientific one. They may simply say, in their own tongue, "Of a location", like how the Bajorans say they are "of Bajor." And indeed, that's what the suffix "an" means. Including the 'an' in Human.
Additionally, not all languages have compatible sounds for the human ear/mouth. While not true of Vulcan, and probably also therefore Romulan, the language of Betazoid is a bit of a mystery. And even in Vulcan, the correct name of the people is altered to match the mythological god by the Universal Translator to make it easier for humans to remember.
Much of how the UT would need to work would require this kind of tokenization of proper names. Many practices are named for people, like pasteurization. Imagine a practice unknown to humanity, and named for a person in a tongue that exists in an audio range beyond human hearing.
It's a matter of language. If the alien is speaking English, then surely he would use the established term "human" for our race. If not, then either it's translated or left as gibberish
If left as gibberish, we don't know what was said. If translated, it's the translator's choice between the alternatives (whether to go for literal or figurative translations). If you go for the former you may have words like earthling, but that kind of translation is usually not considered good.
Korax mentined Earthers in "The Trouble with "Tribbles" and Spock was called an Earther in "Yesteryear".
The name of a planet, region, country or other geographic or galactographic area is a toponym. The name of an ethnic group is a ethnonym. The name of a language is a glossonym.
On Earth there is no rule that the connected toponym, ethnonym, and glossonym of a region have to be related or sound similar.
For example, in the English language the toponym Ireland and the ethnonym Irish are connected but the glossonym of Gaelic is often preferred to the alternate one of Irish.
Many countries and ethnic groups do not have glossonyms.
China is called Zhongguo, the main ethnic group is called Han, and the official language is called Potonghua, or Guoyu, or Huayu.
The frame story of Star Trek could be that actual record tapes of starship experiences were sent back in time to the 20th century and broadcast and thus we see and hear exactly what actually will happen.
Or the frame story of Star Trek might be that information about starship adventures was sent back in time to the 20th century and was used to write the scripts for star trek episodes which were then filmed the same way regular tv scripts were filmed with actors and sets etc. In that process a lot of future English or alien languages might have been translated into 20th century English for the benefit of the 20th century audience.
Thus explaining the use of Humans instead of Earthlings or Terrans.
I remember a science fiction novel where the character Panjarmeeklotutmurph was from the planet Jusaleminopratipup but his ethnonym was the mercifully short Atakit.
In star trek alien ethnonyms based on their planetary toponyms are probably more common than they would be in real life, so it is no big deal that Humans from Earth and Klingons from Kronos (I can't remember the other spelling) are exceptions to that rule. There should have been more exceptions.