Semi-OT: Planetary governments

[Alneyan]

In SEIV, and in quite a few sci-fi universes I have heard of, Earth has become a global government in the future, under various names, and the same could be said of many Empires I have seen at PBW. My question is, would such a development be likely in our future? (Assuming we build an Empire beyond our planet, which isn't a given; but that's another topic I guess)

In the past, while a few Empires were built (such as the Roman Empire, Byzance, the Mongol Empire, and others), none of them managed to retain their full size for more than one or two centuries at best, and they only encompassed a part of the world. The same goes true for the colonial Empires. In fact, there has been an increase of states in the past few decades, and not the other way around, and I cannot think of a successful political union between a few countries (the European Union can hardly be called a success in this regard, although it is one of its ambitions).

So I do not truly see why a global government (holding any kind of actual power) would be our future. A crisis would likely be a way of creating such a government, assuming it is important enough. Or would Earth be represented by a sort of institution for matters related to space? Even if most of the world adopts a fairly similar culture (and that is yet another debate), I do not think many countries would willingly give up their sovereignty, even if they do not hold much actual power, and so the creation of a planetary government as a "fait accompli" does not seem all too likely to me.

So, am I missing something here? Or did I get the wrong idea about how Earth is represented in most sci-fi universes? Should I stop asking questions? What is this huge yellow ship in the sky?