[quote=Dinh AaronMk]
Why not make it in a fantasy world?
[/quote]

We could, but then it's much more freeform, Cultures, races, money, goods, would all have to be made up. If it's in this world we know what does and doesn't exist.
That said, I'd rather do a Fantasy or sci-fi world (say, we left Earth and all converged on a world the governments officially don't know about, as it would result in people like us leaving) than a historic world. I sucked a world history. American history was ok, but only because it affected me.