system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do US cities expand outward and not upward?
gpt
They expand upward when land is scarce or expensive, and they expand outward when the opposite is true.

In most cases, it is the later.  The US is _huge_ and available land is not a problem that we have to deal with.  Most major cities have more than enough surrounding land to expand on to at a fraction of the cost of a skyscraper.

There are a few exceptions, like New York where the value of being on Manhattan Island warrants the cost of upward expansion, but it just doesn't make financial sense for most cities.