I took honors level classes in english. I would argue that constructing a persuasive essay or writing a story is going to be much nore approachable that what I spent most of my highschool english classes dong. (reading shit and writing analysis of it). With that in mind, I don't know if I can agree with the notion that it is modulated to the capability of the median student.
That said, its obvious why taxes are generally not covered. if you're straight out of highschool, you probably have a standard w2 which means 10-20 min filling a return in some tax software. If you run a consultancy like I do and have several sources of income, the knowledge required to do your taxes is too complicated to teach in high school. In my case, my accountant takes care of that.
lastly, we don't teach about fiscal responsibilities, civic duties or the law because they wat us to be consumerist little sheep.
But I had to take 6 chemistry exams in college/Master's, and even though I was in the top 1% of students, I remember virtually nothing of anything I studied related to chemistry. And before anyone says "when you open a chemistry book, memories will flood your brain," I did, for multiple subjects, and I saw no flood, only drought.
EDIT: It might have been due to the curriculum, perhaps if they had chosen authors which resonated a bit more (or at all) I'd have remembered some of it.
In some societies, that ambition, when not "natural," is acquired by proxy from parents, accompanied, sometimes, with a dose of self-hatred, stunted growth, and poor social skills' development. A valuable trade-off for some.
Consider this: if achieving some greater than minimum educational targets was considered by "society", or people who did not get those higher than minimum target themselves, something to aspire to, something that would make a difference in one's life (after accounting for "ambition", inclinations, and intellectual talent), why there are almost zero viable opportunities (and interest!) for adults to make up for lost ground or opportunities?
Some may say, "it is because they have families, less time, work etc.". But how many spend hours upon hours watching football or drinking with their besties or buddies talking about nothing at all?
It is all quite hypocritical. Yes, "society" says that school is good, important, necessary for a fulfilling life, but in practice it is not considered to be so, after mandatory schooling is over. Let's get rid of this intolerable hypocrisy and let people follow their inclinations.
So, the answer to the question "why doesn't school teach [writing, taxes, fiscal responsibilities, civic duties, the law, etc.]?" can be found in the capability and ambition of the median student. And 50% of students are worse than the median student.
This applies to regular public schools, private schools or programs for gifted students have different scopes and expected quality of students.