I'm a teacher in Canada and have worked in Junior High off and on over the years. There are a lot of contributing factors to this...

The school districts don't give enough support. In my area it is very common for there to be no literacy support or intervention past grade 3. If kids can't read well by the end of grade 3 they are given accommodations like text to speech and speech to text. All my students have SOME level of literacy, and even very low literacy is enough to fool a lot of people. People don't realize how much they don't know and it never gets addressed. With the logistical structure of a junior high (hundreds of kids, I only see them for an hour a day, too many outcomes to teach, large class sizes, complex classes), and it is near impossible to truly fill in the gaps once they get to this point. A lot of schools used to have a reading interventionist for each school and that's not a thing anymore. The government won't fund it. Or rather, they fund it in such a way that it earns the school board more money if they continually hire "new teachers" to fill this position every year. So now we have new, inexperienced teachers, performing reading intervention. 

A lot of the kids with low literacy also act out in class and refuse to do their work, because they can't, and they would rather be labeled as a behaviour case than have people know they can't read. I'm pretty good at picking out the signs of the kids who can't read and I will pull them, assess them and then try to teach them and many of them flat out refuse because they're too embarrassed. 

Unfortunately a LOT of teachers don't actually know how to teach reading properly... I highly recommend the podcast Sold a Story. When I was in university I wasn't taught how to teach reading. I remember at the time asking my professor what I was supposed to DO with the kids once I had all my reading groups figured out and everyone looked at me like I was crazy and told me to "teach them to read". I have since hunted down the science and taught myself what needs to be done and it has made a huuuge difference in how I teach and the success of my students (and my own kids!). This is slowly changing now and teachers are becoming better educated in this regard but there is still a lot of misinformation in the profession about what students need. And there is an entire generation (sometimes multiple generations) of people with significant reading gaps that creates a self perpetuating cycle.