I noticed that even these little HN checkboxes have a grammar checker. Earlier, I carelessly wrote "the the", and it underlined the second "the" as incorrect.

And yet, the comments in response to my OP are full of easily detected errors. It's like you have to make an effort to screw it up.

That's your browser doing the checking, not anything that's part of the HN page.

In my experience, spellcheck leaves a lot to be desired. Even on a finished document that I've manually verified as being perfect in terms of spelling and grammar, there's usually still a sea of red and green underlines offering incoherent suggestions in English as dictated by Google Translate.

If the authors of the documents you mention are anything like me, we're so used to ignoring false positives that the net result now permits minor errors to slip through.

As someone who does a huge amount of writing, it's always surprising how many errors still end up slipping through even though I do at least one re-read when I write something. It's basically impossible to do a good proof-reading job on your own work, especially if it's something you've gone through and reworked multiple times like (presumably) a resume.