I think you're right; it's common in the US but not so much elsewhere.Seems much more common in US then or perhaps in older age group. Very common here to use only a fitted sheet and a duvet.
You can buy fitted sheets singly, you don't have to buy them in a set.
"You get two sheets. Every Saturday, you put the clean sheet on the top... the top sheet on the bottom...
in response to this and similar comments - people who don't use top sheets tend not to use blankets but to use duvets with removable covers - these are usually made from washable cotton, same as sheets, and just get washed with the sheet.I use a top sheet…
Don’t have to wash blankets as often..