The people you’re talking about do not actually need to write well. The assumption is that the reader is capable of handling better writing and thus if the writing is better, the reading will be also. That’s not the case, not merely because improving writing often doesn’t improve the experience of reading, but also because most people don’t have extensive reading capabilities.
We test students entering college on their reading comprehension, and in general we only require them to understand approximately 60-70% of what they read. This means that 30-40% of what most people write is pointless crap that can go in the trash. Some idiots who aren’t considered confident in their intellectual abilities might scoff at this assessment, but unless I am reading and writing for an audience that also is capable of understanding the expression, then it is pointless to attempt expressing more.
That’s why I believe you noticed that most people don’t do well in reading/writing… it’s simply not a skill most have and it also is not overly meaningful that is the case. Why paint for the blind?