How do you know if someone can write? It’s true that in modern society we’re predominantly literate to a certain degree of competence. We produce texts for work or pleasure that are readable and understandable. There may be an occasional grammar mistake or misspelling, but, broadly speaking, we can write.
I’m not talking about that. I’m talking about the ability to write with a level of professionalism that the majority of people don’t have. Maybe we can all write an email or a review or a report, but the effectiveness of the text and the ease of reading vary hugely. The longer and more complex the piece, the more obvious it is whether the prose is well written or not.