The human inclination is to believe in its superiority over other *human* tribes/nations, not over demons, dragons and gods that they presumed to exist. That is, the rats believing they are superior over another clan of rats, but dare not wage war on the dinosaurs.
First of all, if the human inclination is sufficient to believe in its superiority to other mortals alone, then that's sufficient enough inclination to explain 90% of the behavior he requires of his nation. The remaining 10% is easily explained by, "We have been wronged." You don't have to insist on human rationality, especially given the abundent evidence of human irrationality.
Plus, while you are emphasizing one difference between the D&D universe and our own, namely the tangible presence of monsterous creatures of superhuman power, you are neglecting to emphasis the other equally important difference between the D&D universe and own. Namely, in the D&D universe humans can legitimately acquire demigod-like super-heroic power. Those dragons and demons and stuff may overawe real humans by comparison, but they must certainly do not overawe the humans of the D&D world and we have countless examples to demonstrate that. Quite unlike the real world, in the D&D world you can aspire to be a god and even dare to make war on the gods. If you think human hubris is great now, imagine when they can become 20th level characters. If you think human hubris is great now, imagine what it is like in a world where you really can make things happen because you will them to happen.
There is also a difference between believing in the superiority of your culture (cuisine, language, politics, etc) vs believing in superiority enough to wage war on the world. The latter requires a real advantage, like the Roman technology and superiority in battle.
Human history shows that anyone who acquires the latter tends to use it to spread their culture, beliefs, language, politics, and systems to their neighbors - for good or ill, with our without the consent of the recievers.
However, they are seeking to wrest the secrets of the universe from those much more powerful. That takes a certain hubris with no equivalent in the real world.
I disagree. When it comes to hubris, real humans have no parallel., and seizing the universe by force if necessary is something mankind begins to fantacize about from the time they are barely able to walk. Every three year old would lead a war on the heavens if they could. Imagine then what it would be like if said three year olds could grow up into to customed sword and sorcery ubermensch? Heck, for some players of RPGs, this is actually the main attraction.