The teacher doesnt read, they skim, and they already know who deserves As or Ds.
I was a victim of this. I was a general A or B student, but I thought the funny kids (D students) were funny and hung out with them. I got stereotype graded. My last paper of the year I completely gave up, the least effort ever. Teacher gave me an A and said 'You improved so much!'
An optimization when I was a student was to find out what the teacher thinks and re-affirm those beliefs with a few twists to give an appearance of depth. On occasion, for fun, I would take a dissenting position and I was always punished for it.
I think the entire education system is steeped in orthodoxy such that it's not in its interest to properly teach critical thinking, failing to do so is an emergent behavior / happy accident. There would have to be an environment that would reward students for actual critical thinking and not apparent critical thinking (agreeing with the teacher) and I don't know how to create one, and I especially don't know how to reform the current system.
I still get a bit of a kick out of the idea that the often proposed solution to the mass academic plagiarism, following the replication crisis, is a mass amnesty - which strangely seems to have tacitly occurred as it's no longer even being discussed.
The teacher likely didn't know that he used to be my best friend growing up, and at some point was more knowledgable with computers than me. He introduced me to things like IRC. But he became one of the most popular kids at school and started distancing himself from me.
After getting our papers back, he came over to brag about how he found his paper online and that's how we discovered we submitted the same exact essay. At that point in time, I thought the teacher must have assumed he copied from me. But I think your explanation is likely more plausible. I guess the teacher just skimmed the papers and graded based on our expected grade.
It got so bad that his Dad, who was an active English and Spanish teacher at another school, was convinced to write one of his papers for him. He got a D.
Then I developed a formula that essentially went, "While {common sense assertion is true}, we need to consider the nuanced implications of {regurgitated pros/cons}." Combined with the smooth fluff and flow from using speech recognition with minimal edits, suddenly the A's started rolling in. I later found this of course works wonderfully with standardized testing essays in the GRE and GMAT.
Edit: I realize now why I get (even if I don't fully agree with) the 'stochastic parrot' dismissal of language transformer models, I basically lived it.