It’s because people misconstrue the debt all the time. The debt is for the kid’s lunches but it’s to the parents not the child. Unless it’s changed drastically since I was in school.
Also, the student is likely to get the meal even if they can’t afford it. Partly so they don’t get stigma from not being able to afford lunch, and partly because the kid still needs to eat.
Still, it shouldn’t cost money for a basic lunch. That’s another thing Republicans have been pushing for. No school lunches, free or otherwise.