The last sentence is it - most people can't communicate much less write well, hell, I don't write well, but I hope my ideas are at least clearly communicated.
This "writing well" as a form of good communication is needed, but while in school, those same people who cannot write well also likely were complaining about learning how to write essays and such. Over time, this sort of lack of learning has resulted in poor written communication into adulthood i reckon.
And with the advent of LLM and all these chatGPT-esque bots writing for them, esp. in school, the level of literacy skill is only going to continue to drop!