Ranches were popular in the 80s and 90s when US boomers were buying "forever homes".

For everywhere and everyone else, the foundation and roof are the most expensive part of the building so it makes the most sense to double or triple your square footage between those by building 2 or 3 stories.

Stairs are for healthy people.

Is it uncommon? I feel like I see them all the time.

We just have a lot of houses, and a lot of space which makes it so you can feasibly have a generously sized single story house.

There's also a trend towards making the basement comfortably livable in a lot of areas.

population density is much lower, so land is much cheaper, and multi story houses are harder to build.

so you just use more land for your house.

Also extreme weather is more common in the US. single story houses are more susceptible to this

Where are you looking? This very well is likely regional. I grew up in the Midwest and every house in my neighborhood had at least two floors, usually more like 3 (ground floor, finished basement, 2nd story).

Communities built for older people often only have one floor due to mobility issues and parts of the country (most of the south) do not have basements. If you’re relying on film/television then it’s likely they’re not filming in the parts where basements are common.

In North America, cellars usually are found in rural or older homes on the coasts and in the South. However, full basements are commonplace in new houses in the Canadian and American Midwest and other areas subject to tornado activity or requiring foundations below the frost line.