One of the factors may be the desire for indoctrination, from many sides. As long as you're legally a child, your time and attention is effectively someone else

One of the factors may be the desire for indoctrination, from many sides. As long as you're legally a child, your time and attention is effectively someone elses's property. The school system can have you schooled for n*10 hours a week on whatever it considers objective truth. There is lots of political tampering with this everywhere. Parents can (legally) control what media you consume, what information you get and who you can interact with. It's partly about the power to make young people what someone else wants them to be. (Not that it usually works as expected.) I'm not saying adolescents should not be educated (in fact, I think there is lots of unrealized opportunity for society in public education), just that this is a tasty quasi-political power that's hard to give up and tempting to expand.