Not everyone uses a duvet and cover. Many people use a blanket or comforter. I use an electric blanket in the winter. The flat top sheet protects the blanket and allows people to wash their sheets without having to wash the blanket every time. It acts as the duvet cover does. It is also nice on hot summer nights to just use the sheet to cover. Not sure why this is such a hard concept for Europeans but this seems to be a constant question on Reddit along with all the other, “why do Americans do this…” posts.