Network effects I suppose. When this economy was based on land and natural resources it made sense to move (and subdue the native population) to open land and gain an advantage, with a service based economy it makes greater sense to cluster at population centers and leverage that. The rise of remote work will probably alleviate this a bit but education, health care, entertainment, hospitality (aka. the fastest growing sub sectors of our service economy) are more valuable when delivered in person. Not to mention fighting climate change (not to say that we're doing a particularly good job) encourages density.