I teach prep English classes at two colleges. "Prep English" means a course for students who have graduated high school, but whose writing skills are behind regular first-semester college expectations.
Why can't students discern that what they write does not match what they are thinking? That is, why do they not realize that the reader will not receive the same meaning from the student's writing as the student does when rereading his or her own writing?
As time progresses, I can identify several issues. One is custom: by college age, every student has developed customs of approaching the activity of writing. Many of these customs are not effective. The student managed to function in high school English well enough to graduate. But the student's customs of seeing a writing project through from start to finish aren't workable at the college level.
Another issue is symbolism. when any person in the world writes, that person has invested meaning into the words on the page. When the person rereads right after writing a paragraph or essay, the symbolism remains strong. Only by not looking at the essay for a significant length of time can the person understand how it comes across to a reader.
But another big problem is the relationship of the sound of language to the marks on the page. Spoken language can proceed adequately with large variations in dialect, pronunciation, and regional word variation. Gestures, facial expression, and tone of voice fill in information missed by differences in language sounds. However, writing has to be standardized if its meaning is to be widely understood. Peepal kant spel wurds iregulearly without causing their readers to miss their meaning.
Here is a major obstacle to many prep-level writers. Reading is one thing; it is another to remember the spelling of every word as one proceeds through an entire essay. At the moment of writing, a student who has writing trouble often not remember how a word is spelled. Unfortunately, English spelling corresponds all too casually to English word pronunciation. A student who is more oriented to spelling by sound than by visual memorization will spell the way he or she hears words in everyday conversation. If that student did not grow up hearing and speaking standard news-broadcaster-style English, then the student's ear will correspond poorly with standard written English,
That student's poor spelling will make the student look ignorant. The nonstandard spelling forces the reader to figure out what words are being used. The student's ear is working perfectly well in using English letters to represent English-word sounds according to the student's daily speech patterns, but that good ear function isn't noticed when spelling is nonstandard. The reader quickly gives up on trying to get meaning out of the student's writing and deems the student too ignorant to pay attention to.
And there exists a tragedy. I have never met a college student who lacked alertness to justice and injustice, comedy and tragedy, moral strength and moral laxity, the importance of personal life vision -- all of these life issues are deeply important to people of college age. If the student's thoughts are discounted based on nonstandard spelling, how profound is the loss!
What can be done to help students recognize the gap between standard English (not always reasonable) spelling and their sound-driven (and often quite reasonable) spelling? That is the professor's problem.