Because it prioritizes exercises that are easy for a teacher to mark over exercises that develop real writing skills. Everything is broken down into elementary mechanical tasks and marked with rubrics. Writing is taught as if it were mathematics, that you can just follow a simple algorithm and you are done!
Turns out, neither great writing nor mathematics work that way. Both are very hard and require non intuitive leaps of inspiration.
Following the algorithms for writing makes you neither a bad nor a good writer. If you learn and follow: tell them what you'll tell them, tell them, tell them what you told them; you writing will be formulaic and boring, but competent. If you do all the things, you'll be an ok writer.
Boring writing is bad writing. If you can't holder your reader's attention long enough for them to get what you're trying to say then you've failed to communicate. If you follow that formula, not only will your writing be boring but too long as well.
Step one, then, is to have something interesting to say.
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.
I struggled with a teacher in high school who expected me to write essays with a formula I could never really master, there might have been other things going on but a big one was that I read so much that I had so many examples to work from that her formula didn't make a lot of sense to me.
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.
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.
I took a two-year course, where our homework was primarily focused on developing an essay at the end of each class (about 14 of them all told). All the classes had similar formats and requirements, and so all the essays also had maximum word counts. I really, really appreciated that, because it surely forced me to distill all my thoughts into the most essential passages and cut out the fat.
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.
