Humans have European or American-based culture, while fantastic races or aliens have a culture based on different humans
Name Space: Main
Page Type: Trope
In many Medieval European Fantasy settings, humans are basically Western Europeans with a few modifications. Correspondingly, in the Standard Sci Fi Setting, humans are basically Americans with a few modifications. What about the other fantasy races and sci-fi species? Well, Starfish Aliens might have a totally alien culture -- or might have a biology which renders the notion of "culture" itself dubious (such as a Hive Mind), as might some rather alien fantasy races such as an Elemental Embodiment. However, for Rubber Forehead Aliens or the humanoid Five Races and Fantasy Axis Of Evil, often their culture will be based on some non-European or non-American human culture.
Effectively, the cultural diversity of humans is streamlined to some roughly European or American culture, but human cultures which don't fit the European or American mould are re-assigned to non-humans. Subtrope of Fantasy Counterpart Culture, which includes more cases where the non-European counterpart culture is human, and of Write What You Know.
In Eberron, the orcs of the Shadow Marches seem to have an aesthetic and culture similar to Vietnamese peasantry, tending swampy rice paddies in conical asian-style hats and living in thatched huts. Khorvese humans are largely European.
In Warhammer, Chaos Dwarves seem to be vaguely Babylonian.
In Age Of Wonders I and II, the Azracs and Nomads, respectively, seem to be culturally Middle-Eastern. Humans seem vaguely Western European.
In Wing Commander, the Kilrathi have a resemblance to Imperial Japan. However, in the case of the humans, this is largely averted, with humans hailing from all of Earth's cultures from American to Maori. It's still somewhat American-centric, but hardly exclusive of other aspects of humanity.