I forgive Star Trek for having aliens that all share common cultural traits because at least some writers recognized the show did that and try to explain why it's like that. Several alien species mention that they had massive, apocalyptic wars before developing space travel. Vulcans did it, Ferengi did it, several others too, including humans. They then had to rebuild, but it always ended up uniting the planet under a common ideology and culture over thousands of years. Humans are the weird ones, we develop utopian space socialism in only a few decades after global nuclear war, whereas Vulcans took 1500 years.

A few episodes try to say it's because Earth is more ecologically diverse than other planets with alien life, like the Vulcan planet is a huge desert and the Klingon planet is covered in uniform volcanoes/mountains. A few try saying humans have much squishier brains than other species and that results in more adaptable thoughts.