US schools do not always have cafeterias and many students don’t use them. They cost money and bringing a lunch is cheaper. I know there are movies/tv with kids bringing their lunch and trading items as well as buying things from the cafeteria. Sometimes its setup were the parents paid for them to get a meal every day with a kind of pass or another style where its kinda billed to an account. So im guessing you have seen media that made you think kids always use a cafeteria but man you someone avoided the ones showing kids bringing lunch. Anyway some kids use when there is one and some don’t.