Regions and cities do not build up evenly, but rather unevenly as talented people and businesses cluster together in close connection.
Here’s a partial quote from the longer post/article by Ryan Avent:
The value in economically dynamic cities is the people that populate them. Where once, firms would pay high land prices to be near coal deposits or harbors, based on the economic advantages those amenities conferred, they now pay high land prices to be near talent. This yen to concentrate in particular areas has a number of dynamics. Firms want to be near customers and clients. Workers want to be near firms. Firms want to be near workers. Where there are lots of firms and workers, there will also be businesses serving those workers — in business and in the provision of consumption opportunities — and those services attract additional firms and workers. Everyone wants to be where everyone is, and it’s tough for anyone to go somewhere else because somewhere else is where people aren’t.
The result is an urban geography that’s very lumpy. People clump together, because there are gains to doing so.