I think we often forget just how many of us there are on this planet (or even in a given country). Each of those million people all had their own homes, lives, businesses, and so on. And as the country sees population decline, much of that just gets left to rot. Homes that have been completely abandoned, usually because the owner died and left no heir, even have a term - akiya. Here's [1] an image search with all sorts of them. They make up more than 13% of all homes in Japan now. They have more homes than people to live in them!
And if things don't suddenly turn around in dramatic fashion, soon there will be tens of millions of these sort of places. In some you may even find the old owner. There's also a term for that - kodokushi [2].
[1] - https://search.brave.com/images?q=akiya%20houses&source=web
My own hometown has experienced rapid growth over the same time span as Nashville, and has annexed large portions of what were commonly known as the suburbs. So there has been a literal change from what used to be suburbs to now be within city limits, in the course of a generation.
The incredibly low population density you can find on Wikipedia reflects the sprawling suburbia that composes the "city."