Why schools use the methods they do
Most of what we believe we know about how learning must happen comes from the decisions schools make about education.
But schools don't make the decisions they do based on what's best for children. They base their decisions on what's best for schools. And that makes sense. What's best for the schools is what's (seemingly) best for children since if the schools can't operate, children can't (supposedly) learn.
But schools are also assembly lines. Assembly lines are cheap and efficient. Most teachers would agree that one-on-one and small groups are best for learning. Yet children are taught 30 at a time because it would cost too much to have one teacher for each child or even each half dozen children.
Having large class sizes is a huge compromise in what children need to learn but it's done for practical reasons. What's gained by having schools that can operate (by not costing too much) seems better than not having schools at all.
So kids are taught assembly line fashion because it's cost efficient. And because assembly lines are necessary for schools, schools make the decisions they do that keep the assembly line functioning smoothly.
Children don't need to read independently by 9. Schools need children to read independently by 9. (And what's best for schools is (supposedly) best for children.) Schools as they are can't effectively help children who can't read independently by 9 so all children must read by 9.
That goes for everything that schools teach. Children don't need biology at 15. Kids will acquire and draw on biological knowledge when they need it. But to run efficiently schools need to have all children learning biology at 15.
So how can they learn biology and algebra and everything else unless it's taught to them?
The same way they learned English: as a tool to get what they want. By using English to get what they want they get better at it as a side effect. And the beauty is that it's self correcting because the goal is to get something that works rather than to acquire something that's right.
If we think of how they learned English versus how kids (supposedly) learn a foreign language in school, which worked better? English is learned practically effortlessly. But we don't take that learning seriously as real learning. It seems some magical process that applies only to very young children learning their native language.
If we assume the only legitimate knowledge is what's in textbooks, and the only legitimate form it can take is what's in textbooks, then it's going to be hard to understand unschooling. Few people will be encountering and solving 2x + 3y = 27 in their daily lives. Few people will absorb the important factors of the Teapot Dome scandal by living life. (Those who do need it because of their interests, though, will be getting that.)
But textbooks are like the details. And real learning is about building up an understanding of how the big picture works. Schools hope that kids will understand the big picture by cramming in the details but if it happens it's a side effect. But unfortunately schools are handicapped by having to prove to administrators and parents and the state that learning is happening. So, despite wanting something better for kids, schools need to concentrate on the type of learning that can be demonstrated, that is the kind that is testable. It's hard to test understanding so schools settle for testing what can be memorized and hope that understanding happens as a side effect.
So kids in school learn the details and might gain an understanding of the big picture at the same time. But it's hard and often not successful. It's hard to grasp the concept of what percentage means in a larger context, when immersed in figuring out 37.2% of 128. But by absorbing the big picture, by slowly absorbing when and why percentages are used in real life contexts, then the details fit easily into the big picture. And by absorbing from real life, they aren't picking up negative associations with percentages. Percentages are just a useful tool to them.
The world is a naturally fascinating place! Kids want to explore it. There isn't a reason for a child to avoid learning fascinating things. Unless someone gives them the idea that learning is something to be avoided. Unless someone packages up the world into dry textbooks and locks them up in rooms and makes them memorize it.