There are different types of learning. It can be easily said everything a person learns in school can just be learned by reading a book outside of school. Howev

There are different types of learning. It can be easily said everything a person learns in school can just be learned by reading a book outside of school. However, some people learn better by lecture and other people by simply reading on their own. There was also a movement in the 90s with secondary education where teachers tried multidisciplinary education -- I don't follow these things so it might still be a thing now. The idea is students are more engaged and retain information when instead of being taught math and history separately, they are taught the history of Fibonacci as the same time they are taught about Western Civ and the dark ages of math in science in Europe. Another example is integrating metal shop with physics no becoming energy shop. What OP has discovered is that what might have been boring and inconsequential to his life now has become important to the task at hand.