In my country, there’s no US style cafeteria (instead it’s a canteen where it’s up to the student if they want to buy anything) some bring their own lunches (home or leftovers) to school, so why do American kids rely on cafeteria food instead of bringing their own?

I mean, is cafeteria food across American schools that bad? It depends on where one resides or if they attend a public or private school. In my case I went to a private (Catholic) high school and the food there is actually good (& cheap when converted to USD).