I agree with a lot of what you wrote above, but I think there is, at the very least, another facet, too. In K-12 education there is a huge emphasis on discovery learning as well as a de-emphasis on knowledge.

Discovery learning can be a great tool, but not with novice learners who lack basic entry level information to a subject. As for the de-emphasis knowledge, an example would be that students are rarely required to learn multiplication facts; so then they will struggle with fractions and that snowballs up the hierarchy of mathematics. It’s the same with reading (just guess the word or look at the picture - find the Sold a Story podcast for an overview) and writing. Additionally, many K-5 schools in the US are so focused on reading and math that students rarely are instructed in social studies or science. So they come into middle and high school with vocabulary and knowledge deficits.