They don't grow hair on their chest neither. Yes it's genetic, but not unique. Many African population cannot grow beard neither. And as we are at it, many Asians cannot neither.
For Jarhead and others:
In Civ3, we had the Iroquois. In reality, the Iroquois were an empire of 5-6 tribes which settled cities. At the arrival of the bad white man, the Iroquois had fortified 30,000+ cities. So the nation made some sense in this game. However, their portrayal in the game was inspired a lot by the Sioux. I remember their special unit was a replacement for the mounted warrior, which made little sense to represent the Iroquois, as this unit was really from the middle plains.
In Civ4, they made them the Native Americans instead. Their special unit and special building makes more sense like that. However, though they are based on the Sioux (the leaderhead being Sitting Bull), they mostly ressemble the middle plains inhabitants as a whole. At this point, they make little sense in this game, as these people did not settle cities. They were nomad and they were mostly always on the move, following their prays (notably the buffaloes). They did not have a concept of land ownership. As they are represented in this game, they seem more like a collection of many of the middle plains tribes.
Of course, if they had been given time (assuming they had not been invaded by us, the bad white guys), they might have evolved and settled somewhere. They might have learned agriculture and settled cities and expanded. After all, the natives of the South America did so. The Inuits (which were not troubled by us, given that they lived in a land which we did not want) eventually settled and found cities (today they even have their own Canadian territory).
Anyone interested in this topic should watch Dances with Wolves, with Kevin Costner. A 1990 movie, one of the last few to be filmed entirely on high quality 35 mm. A very high level of production quality. Obviously romanticized, but otherwise, in my opinion, quite a realistic enactment of how they really lived. Get the 4-hour director's cut Blu-Ray if you can, you won't regret it.