The thing is that by the third grade or so, you probably knew why this is bullshit. Every kid is different and picks up information at wildly different rates. Yet you were all hearing the same math lesson, despite the fact that one kid in your row was crying tears of frustration while another cried tears of boredom. So who decided this was the best way to do it?
It turns out that it was concocted nearly wholesale by a committee of ten guys way back in the early 1890s. They were imaginatively known as the Committee of Ten. Before then, schools across the United States were essentially free to teach kids however they wanted, with very mixed results. So in 1894, the Committee of Ten came together with the goal of standardizing the education system across the country, and released a report that shaped the modern concept of schooling in the USA and abroad. That part makes sense -- why not figure out what works and apply it everywhere?
From here on out, go ahead and assume finger quotes every time the word "works" comes up.