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Personal note: once something become common, you really stop caring about the single units: the USA are 50 states, how many people know all of them, where exactly are they on the map?
First of all, while there are some similarities by region, the states that comprise the USA are wildly different in geography, economic structure, natural resources, and industrial focus. The very size of USA allows for some pretty extreme diversification and focus. Unlike smaller countries, there is no need to encompass all of the nation's endeavors within a single region (i.e. a single state, .vs. the small area of a smaller country).
Second, I think OP's argument (and certainly mine) is that---as planets exist in Stellaris right now, you are right. Once we have a bunch of worlds, they're all pretty much the same and it doesn't matter, and therefore---it would be an improvement if there were a wide enough array resources and building options that we could, in fact, really specialize planets. The ability to specialize our worlds would, by its very definition, make planets special.
So ... your example of the USA is upside-down, and is in fact exactly what the OP is asking for.
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Probably you can if you are american
.... if only that were true for the majority of my fellow citizens =(