Because it's already hard enough to get someone to understand what you're saying if you write full, flowing sentences.

Back when I did customer support, I occasionally dealt with people who lacked the capability to express themselves in written language. These weren't people born in other countries or people with some kind of intellectual disability, often they were just never taught how to read/write/compose a message.

There's also a social stigma to sending a job application written like "good. morning. sir i. wnt too. Wrk for ur Compny u. hve job. for Me im steven i.m" and so on, even if the job in question has nothing to do with any form of written communication.

Plus, there are plenty of things people just don't know how to do. I once had to Google "how do I write a letter of resignation" and Google will now generate a fulll message for you if you type "i want quit next month how write".

I think the author of this blog post assuming a prompt to help improve a message (obnoxious glowing button aside) somehow implies that Google thinks they're stupid showcases the perceived link between the ability to write and intelligence.

Obviously, using LLMs for this is just patching over the lack of resources put into (adult) education and literacy, as well as a societal shift to stop treating illiterate people badly. However, structural improvements take many years, probably several rounds of elections, and doesn't help someone today.