how completely ridiculous it is to ask high school students to decide what they want to do with the rest of their lives

One common answer to that is to become a dropout, try a career or two to find out where your talents really lie, and then go for that. You can usually go back to school for an education when you've figured which one you need.

It doesn't even seem as if it would be very hard to build that right into the system. Doing it the artisanal way takes longer, generates more stress, loses more income.

Tentatively, thinking of my own experience, I'd point to the competitiveness of the system as the driving force. I had some smarts but school didn't suit me much. There were a bunch of things I was interested in - computers, AI, writing sci-fi, evolutionary biology - and I had no clear idea what I should do when I turned 18.

My parents' reasoning was "Most of your interests are scientific, so, the best way to keep your options open is to enrol in the top engineering schools, then you can have your pick of careers later". One problem with that is that these schools aren't a place for learning while you keep your options open. They are, basically, a sorting process, getting students to compete and ranking them so that they can eject the bottom tier, direct the middle tiers to various jobs and the top tier to yet another sorting process.

The material is taught more in video-game order than in the order which would optimize for deep comprehension - that's what turned me away from math. And only that material is taught which makes for an efficient sorting process.

Not that any of that is a new observation - "schools aren't about education".