Because people, especially kids in their formative years, need free time to build their personality, make experiences and grow as a person.

True. Sadly, many don’t get that time after they’ve done glues off homework and mandatory extracurriculars every day and twice on Saturday.

Large part of the reason the school system is what it is today, historically, is that employers did not want to pay for training and education of their own workers (takes too long and is too pricey), and instead decided to put responsibility for that on the public through the school system. Long hours, military-like discipline, so many years to go through, and commonly about topics and skills that should be early professional training, not something anyone has to go through regardless of job.

The idea that you’re an adult with 18 is historically absurdly late, given that humans reach puberty around 14, and it keeps getting pushed higher by the ever-growing demands of employers not wanting to pay for their employees’ training, practically demanding fully experienced workers for entry levels, and parents and the public to be responsible for that. Now with college and all, many Americans are of the opinion that you’re not reeeeally an adult until 21, or 23, or 25 and sneer at the idea… well, no wonder, because maturity comes not with age but with experience, and if you shelter and restrict people for X years while they are in school and legally a minor, they will grow up once they’re out of that, no matter if they’re 14 or 18 or 21.

The geographic differences are particularly crass. If you ask an American whether a 23-years-old “college kid” is a real adult, they will most likely consider them barely grown up naive kids, but in Europe, where college/university is actually not an infantilizing expansion of the school system but adult education, people are clearly much more mature. When I started going to uni around 18, I rented an apartment from a private landlord, had to care for my own insurances, taxes, bills, groceries, finances, everything… while my American friends were still being cooked for in their dormitory, with an enforced bedtime, rules about who can do what in their dorms, weird secret societies, drama, the “school spirit”… It felt so childish and infantilizing - but guess what, they mentally stayed teenagers until they graduated and got in touch with the real world. It’s all nurture, barely any nature.

People used to be adults around 14 or 15 throughout history, until capitalism came along and set unreasonable expectations for what parents and the school system should do. The fact that kids’ lives are all taken over by school is by design.