Adding onto that, why are there cases like Seborga and Goust, which were never formally annexed to Italy and France, respectively. Were they just forgotten?

Yes and no.

Nobel laureate Douglas North was one of the most prominent economic historians of all time; his 1999 "Understanding the Process of Economic Change" still is one of the pivotal texts examining the root causes of economic growth. He attributes Western Europe's economic primacy as a consequence of a geopolitical landscape characterized by medium-sized geographically defined states locked in fierce competition with each other.

Why was Western Europe was locked in a spiral where small(ish) states were in fierce competition to develop efficient and effective state and non-state institutions? Indeed, why is it that after the fall of the Roman Empire, all attempts to unify Europe were short lived and ill-fated?

At the same time as fall of the Roman Empire in the fifth century, on the opposite end of the Eurasian landmass the Jin Dynasty had also collapsed, giving rise to the “Sixteen Kingdoms” period of Chinese history. Why is it that by the late sixth century China was soon again united under the Sui Dynasty while Europe sustained continued political fragmentation? The question is especially poignant considering most estimates of economic development place the China ahead of Europe at the time (the most notable of which were composed by Angus Maddison at the University of Groningen, last updated in 2013).

The answer is that Western Europe, after being united by the Roman Empire, adopted a robust legal code regulating justice and legal enfranchisement, emphasizing local rule and dialogue between government and governed. The Roman system was rooted in the Mediterranean tradition of government stemming from urbanization: given the superior convenience of sea travel until the invention of the steam engine, the European continent (which envelopes the shallow North, Baltic, and Mediterranean Seas) is particularly favorable to the formation of cities, with swift trade routes passing through fertile areas which can easily support large populations that aren't farming.

A government system based on efficiently governing prosperous urban spaces is not only very well-disposed towards defending compact and well-defined borders, but begins to break down when the area it is accountable for becomes too large or too diverse.

In short, Western Europe was geographically, economically, and socially predisposed towards developing and maintaining highly competitive societies with institutions favoring enfranchisement fermenting the creation of nation-states.