Unfortunately, not all kids that go to school can afford to pay for food. In some cases the meal they get at school is the only one they get.

Source: Worked with public schools

This has to be reiterated whenever anyone talks about 'American schools.'

US has a Federal system where each state runs its own schools, and each local community has its own school board. Two schools can be ten miles apart and have completely different standards and curriculums.

Also, each school has a different social milieu. In one place, the popular kids might bring in meals cooked by their maids, so eating cafeteria food is a sign of poverty. In another place bringing your own lunch might be a thing only the poor kids do.

In my school about half the kids packed their own lunch and the other half would get a lunch from the school. The school lunches were made hot and you had a rotating menu with 3 options and a salad bar each day, they called it hot lunch. Some kids (most of the ones who got hot lunch) received it at a free or reduced price. A little over 2 dollars normally but 75 cents reduced or free like I got mine. Some schools did have a canteen rather than a lunch menu. American culture is not anywhere near as homogeneous as it appears in our media.

When I went through school, I did both at various points. Sometimes I'd buy lunch in the cafeteria. Sometimes I'd take a bag lunch.

The cafeteria can do hot food, which you can't bring. But a bag lunch lets you choose options that aren't served in the cafeteria.

I mean, honestly, same subsequent to education, at work. People might bring a lunch, or might eat commercially-prepared food. Not really a universal standard.

So, I'm not sure I understand the question - you said your schools have canteen and kids use them.

What reasons do they have?

The cafeteria at our school is free for everyone.