Locales differ but in my experience:

1. no one is required to eat the cafeteria food
2. the cafeteria food has a cost
3. the cafeteria food quality was low when I was young but not terrible, and is much improved now
4. many kids absolutely do bring their own lunch. My kids look at the monthly menu and decide if they want to bring something from home or eat what’s being offered at the cafeteria

To be honest I find your question confusing. It seems to start with the question of whether everyone has the option to buy items or eats at the cafeteria, but then jumps suddenly into “is the food that bad.” I don’t honestly understand quite what it is you want to know.