I did reconsider the use of using the word "Deaf" several times, but in the end I left it in.
It is true that some deaf and hoh people speak "normally", at least from a hearing, native English speaking person's pespective. It is also true that some deaf and hoh people are not native English speakers. They use ASL, which, quite differently, has it's own set of rules.
As N4E pointed out, there are a million reasons why deaf literacy has such a poor showing. In part, I believe that this is because the text deaf asl-users are reading are not for them. If you will allow me to steal N4E's example: If, growing up, you only spoke English, then I put you in a school where all of a sudden nothing was written in a familiar language, you would be pretty confused when it came to this next language. Then, even though you weren't developing literacy skills (again, from a hearing POV), you'd probably simply graduate year after year. "Good enough."
There are also simply not enough educated teachers out there capable of teaching deaf asl-using students how to write in English. This is why, as you will find, much of this website is written in a language unfamiliar to myself, perhaps you and others as well.
This is also why deaf people can carry a stereotype of unintelligence. This is, as most stereotypes are, simply untrue. If you grew up speaking English, but then told you that the only way you could be smart and express yourself was by speaking fluent French, which was simply introduced to you, would you be capable of doing so?
This thread, this forum, this website, this culture.
It is a learning experience for everyone.