It's not really the context that's useful in learning, but the content. Sure, the simple stuff is what we definitely need in life, but the rest is about HOW you learnt it. For example, History is pretty much useless in the long run. However, it teaches you how to analyze, explain and infer from sources. English gives you more language to use, and teaches you how to write a variety of texts. English and Theater improve your speaking and presenting skills, and English along with Art helps to broaden you imagination to create ideas of the future. Languages help you communicate. P.E keeps you fit. PSHE teaches you morals and educates you in how to avoid real world problems and how to excel in life. TOK helps you to debate and stand up for yourself, which you would need if you were ever in question. Science is what we know about life; how would you not want to know it? It stops you from thinking things like pollution is a myth and can help save the planet. Geography teaches you about the world and why you shouldn't be a racist numpty. Economics and Business - do I even have to explain?
Of course reading Romeo and Juliet is useless. But you need a base before you can build.
They don't let kids branch off at an earlier age because we're not mature enough yet. You remember being small and wanting to be a pilot or an astronaut or a CEO right? Well, that's before you realize that few people get those jobs. I mean, my dad admitted himself that when he was small he wanted to be an engineer, and on the last day of college he decided to be a doctor. If it wasn't for that decision, he wouldn't be the big CEO today or even built the company. Had they steered him off as a kid to become an engineer, would he have the life he has today?
Bill Gates and Steve Jobs. Pretty sure they wanted to be a fireman or police or whatever as a kid. They only really got involved with computer at around 18. Imagine if they just kept with their childhood dreams; no Microsoft, no Apple.
Even kids in 8th grade aren't mature enough yet. I'm in 8th grade too. I know I'm not mature yet. Heck, we definitely should NOT choose in our early teen years. That's when we go through puberty and have a hectic and emotional time. When we're 18/20's, and have a heck ton of knowledge and skills balanced in each category, our minds generally become much clearer and we're more mature to make a lasting decision.
Your parents didn't need to know what we do now because they didn't have the technology. Our world is growing at a fast speed now, and there's no real reason to let is stop.
If people just learnt what the previous generation learnt, we'd go hardly anywhere. Imagine a grain of sand. That's the human civilization. Water is our education. If we learnt the same thing over and over again, we'd be as slow as a lake, and that little grain of sand will hardly move. However, if the currents keep pushing us, we go further faster.
Of people stayed by your logic of keeping the same education, we'd be pretty much in the Bronze Age right now.