James of Grognardia is trying to start another bandwagon with this post.
At one time I went through the Monster Manual, Fiend Folio, and Monster Manual II and noted all the possible culture forming races. There was a huge number well over a hundred!
I thought this was a bit ridiculous to try to come up with anything plausible out of. But rather than just cut and pare this I tried to think of a way of making this exist.
By then I already had established an Uttermost War and that it was a war between the Gods and the Demons. The demons were not a special group but rather gods and mortal who revolted and decided they knew how to run creations better than the original plan.
So I decided there were only two races in the beginning elves and human. The elves were created to be guardians of the wilderlands and their ultimate destiny was tied to its fate. Humans were destined for something beyond and their time in the wilderlands was just a step to their ultimate destination.
But how do I get from two to over a hundred sentient races?
It the demons fault. The demon were largely comprised of gods and elves. Humans that came under their control were enslaved. Unfortunately humans being humans made for poor slaves by demon standards. Either they rebelled or broke too easily when driven. So the Demons decided to create the prefect servitors.
They used magic to twist humans into new races. The first batch consisted of Dwarves, Gnomes, and Halflings. With them they were focused on physical hardiness (Dwarves, Gnomes) or reduced need for supplies (halflings). But they had too much of the human spirit and still caused problems.
So next they experimented with animal hybrids the result were things like centaurs, lizard men and the like. The results were often strange and for many there were breeding issues.
Finally they realized they need to go after the mind as well as the body. So they created the Orcs, goblins, and the rest of the humanoids. For goblins they tweaked them so that had a obsessive attention span. They would focus on a task to near exclusion of all other things.
For Orcs they greatly increased their aggression levels and introduced a dominance mechanism based off of that. Orcs will readily follow any leader they perceive to be stronger than they are and their in-fighting left them too divided to think of resisting the demons.
After the Uttermost Wars the Gods tried to integrate demon created races but only succeeded with the earliest that did not have their minds effected. Unwilling to commit genocide they scattered the other races to isolated regions of the Wilderlands. Of course as millenniums passed eventually they all came into contact with one another again.
I felt this explains the AD&D setup rather nicely. While the story of the orcs is tragic, it is understandable why there is so much bloodshed with civilizations that border them.