I think the title is a tad misleading, since it's not so much about humans being objectively superior in all ways. If you go back enough, you do find stories like that, because John Campbell liked them and published the biggest SF magazine in the world at the time, which influenced the genre a lot. It was the drop in popularity of the European racial narrative in the '60s and on, I think, that really changed the most common theme into "humans are good at some things but not at others," as opposed to just being The Best.
Though even back then there were exceptions. Doc Smith's Lensman
setting has enormous numbers of alien species, many of which are superior to humans in nearly every way- psychic power, resistance to the lure of corrupting influences, physical strength, senses humans can't even imagine, and so on. Even while the human protagonists are so exceptional as specimens of humanity that it stretches modern suspension of disbelief to the breaking point, they treat various alien peers and rivals as very much their equals.
Star Wars According to Borsk Fey'lya, the defining quality of humanity has always been our insatiable lust for power. Everyone knows that. I think there's a fair bit of revisionist history going on with Fey'lya's administration, but it's amusing to think that others would view the human race that way.
Since Fey'lya is one of the most scheming, power-grabbing politicoes in his setting, with a truly epic knack for shooting his own side in the foot rather than allow anyone else to succeed at his expense...
He is not even remotely qualified to call anyone
else out there power-hungry. Well, not anyone this side of Gharlane of Eddore.
In video games it's arguably worse than television - humans are generally defined by as the 'diplomacy' guys (ludicrous on the face of it) or the 'trade' guys (which is why we destroy our own economies). Of course, this is because diplomacy and trade are great things to have bonuses in, especially compared to 'shoots a bit better' and 'whoa gets a bit more rocks'. The fluff ALWAYS has humans being PLUCKY and PUNCHING ABOVE THEIR WEIGHT and UPSETTING THE BALANCE OF POWER. Why?
AMERICA. That's why.
I don't know: I mean, if AMERICA was the reason, wouldn't human dominance over everyone else just be assumed since obviously all humans are SpaceAMERICANS? The idea of humanity as the mercantile and diplomatic specialists who are flexible and can deal with diverse cultures and therefore come out on top sounds very un-AMERICAN to me. Almost... French.
We won't give up and be slaves no matter how outmatched we are
Remember; science fiction is not about America sees itself. AT ALL.
Why is "not giving up and being slaves" uniquely AMERICA, as opposed to being a trait of, say, the Polish Resistance? Or, heck, Tamil Tiger suicide bombers? America does not have a monopoly on bloody-mindedness.