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.
To start, there's the fact that they didn't really make sure it worked first. The committee's report basically turned education into an assembly line. Unless you're an Einstein-level prodigy and get to skip a grade or do really, really badly and get held back, your progress through the school system is more or less decided on the time spent sitting at your desk -- one year equals one "grade." It'd be like an RPG in which leveling up didn't depend on how many orcs you grinded, but simply the number of hours you sat at the computer.
"I'm gonna grind through Junior Prom, then I'm never buying anything off Greenlight again."
Indeed, the Committee's standardization model (which was later refined by the Carnegie Unit) wasn't intended to maximize the quality of schooling so much as its efficiency -- they wanted to "batch process" kids for higher education. After a set period of time, you either memorize enough facts to go on to college or university, or you don't (in which case, ha ha, fuck you). And this 120-year-old system relies on the ability of every kid going through puberty to fully understand the weight of their own mortality and forward-plan the next 50 years of their life in an age in which free Internet porn exists.