
I’m no expert on Japanese culture and this is mainly based on other manga I’ve read + googling but I imagine it’s mainly just the high standards of Japan have nutrition. Convenience food, while healthy in a way, still contains food preservatives and stuff and apparently they take children’s lunches really seriously (pretty much to an artistic level) and not following through would mean you don’t care enough for your child’s health and upbringing. In further detail, the artistic value of the lunches allows the children to overcome their pickiness of certain foods and have a habit of enjoying them as they grow up.
Of course it’s kinda one of the common nonsensical rules about Japan that (while has good intentions) is almost based entirely on the way the public views you, like how salarymen go home super late at night just to show they work hard while ignoring the quality of work.
Other reasons for the bento box situation the kindergarten’s many incentives to force the mother to challenge ‘motherly’ activities (this might also be related to low marriage/birth rates). They might also be more worried about drawbacks like parental and child complaints e.g. a child starts to complain that they want convenience store food or their mother doesn’t care enough to cook for them when they see other kids with homemade lunches. Also, while a bit of a stretch, it could also be a food package waste reason. Anyway from my understanding those are all the reasons I can think of but I’d enjoy seeing other people’s ideas.

Spain. Well, I said that taking into account my experience, in another place in the same country may be is different. I've been in a "State-controlled school" (I look up linguee, how it's said in English) and children from 2-3 to 15-16years old eat there, obviously parents pay for the meal but it's the school who is in charge of cooking. Some of my classmates had their meal made at home but they also eat in the school (there are fridges and microwaves in the dining room) and a very few people went at home to have lunch. (sorry for my mistakes :'v, I tried to do my best writing in English)
Can someone explain why the kids can't bring store bought food to school? Like I'd understand if it was already open or something because that prevents a hazard, but why won't they let children bring sealed store bought food?