School actively teaches people to be bad writers.
Turns out, neither great writing nor mathematics work that way. Both are very hard and require non intuitive leaps of inspiration.
Unfortunately it isn’t generally the case that people have something interesting to write at length about, and even more rarely do they have something interesting to say about the specific topic assigned.
Even though I struggled to get a "D" in her class I am certainly not a bad writer, I've made a living at times writing, I've written book chapters, blog posts, contracts, standards documents and all sorts of things. I struggled to master the language used in academic writing though.
https://www.youtube.com/watch?v=vtIzMaLkCaM
Essentially, students are, from the time they start school until they graduate, taught to write to an audience – teachers – who are both:
1. Paid to read it, and have no real possibility to stop reading because they get bored (teachers will read anything from beginning to end).
2. Looking for, in the writing, evidence that the student has learned the information.
Neither of those two things are true of the rest of the world, when they are reading a book or an article by an aspiring professional writer. Therefore, writing to such an audience will require new skills which most people have never practiced.
Occasionally, I would be plagued by a bit of writer's block, and I couldn't think up enough to write, but eventually I could gin up enough logorrhea that I would have a nice tapestry to begin snipping out the wordy and irrelevant bits.
And that was my favorite part about the classes, being able to write a really tight research/opinion paper every few weeks.