They are, but adventurers are the exceptional small percent, and the exceptional small percent is much more mixed in of exceptional beings from all walks of life and speices of creature, rather than the extremely populous peasantry.
So, when we have a cast of five, and more than twenety races to choose from, and they choose to give us, of that five, two and half humans, one and half elves, and a token alien... it just seems like a really bland, boring, uninspired choice.
