Perhaps to my poor choice of words you have assumed that selection is being made based on good writing. it's not that simple.

We have to define "good writing". Good doesn't necessarily means CV written in a way for Shakespeare to become jealous to lucidity. It has to meet bare minimum - as in have zero grammar mistakes.

To make an example, you probably won't go to a first date in your pyjamas with stains on it but will dress nicely. The date (and whatever follows) may not be up to your expectations, but if you do use pyjamas, it most certainly will. Not because your pyjama is not magnificent but just because having negligence in dressing properly shows arrogance and disrespect to someone who joins you.

Well, you see, we need Windows admins, and we find that people who can master the convoluted nuances of the English language are naturally suited.

If you need a Windows admin that states proficiency in Wnidows in the CV, you can infer that not much attention has been put on the CV. If not much attention has been put in the much important first contact with your company, what’s the experience going to be n months down the line, with e.g. powershell scripts that don’t come with spellchecking?

Mostly, hiring has a biased risk matrix. You don’t get hurt by missing a good candidate so much as hiring a bad one. So that makes you conservative in choices. Obviously 1 typo can happen but think about 5 to 10. If you have a lot of CVs to wade through, awful documents going to the bin is not a bad first step in the process.