It could be a preference from video games, or maybe it’s because the common usage and being a synonym for ground and dirt has made the word seem ”low-brow” and the alternatives classy, exotic or poetic, and if their popularity was reversed we would think "Earth" was a classier word, but still.
“Terrans” will always sound better than “Earthlings” however. :)
It is a common word in many languages and it has many forms like Ard or Erd or Jord.
The other word for earth that is also very common is zameen, but that means land.
Earth comes "from Middle English erthe, from Old English eorþe (“earth; Earth”), from Proto-Germanic erþō (“earth”), from Proto-Indo-European h₁er- (“earth”)."
Also (from the page for lowercase "earth"): "Probably unrelated, and of unknown etymology, is Old Armenian երկիր (erkir, “earth”)). Likewise, the phonologically similar Proto-Semitic *ʾarṣ́- (whence Arabic أَرْض (ʾarḍ), Hebrew אֶרֶץ (ʾereṣ)) is probably not related."
It's hard to hypothesize about whether this could be reversed in an alternate universe because this tendency goes back many centuries: for a loooong time Latin was the language of the scholarly and political elite, whereas Anglo-Saxon Old English was used by commoners.
Broadly speaking, the short words are the best, and the old words best of all. - Winston Churchill
French was too for hundreds of years even up until now and maybe for the EU again after Brexit.