I mean, we don't have any mountain ranges in Chicago. We have a lake front, but that only blocks off the east side. What do Ubi?
I really don't want them to implement some kind of killswitch where Aiden has some microchip in his head that explodes if he leaves a certain radius. Or cars explode after leaving the play area.
And now that I think about it, I can't really think of any open world games that don't take place on an island. RDR gets away with it since it doesn't have any means of flight, which means they can just surround the area in mountains and steppes.