From Don Boudreaux at Cafe Hayek, quoting Jane Jacobs' book Cities and the Wealth of Nations:
Nations are political and military entities... But it doesn’t necessarily follow from this that they are also the basic, salient entities of economic life or that they are particularly useful for probing the mysteries of economic structure, the reasons for rise and decline of wealth.
So for example, two important factors in development are technology and culture. Neither spreads primarily at the unit of the nation.
This is the kind of helpful insight that will DEFINITELY have NO impact WHATSOEVER because it is so much more convenient both data-wise and politics-wise to focus on Nations.