A rejection letter conveys a single fact because you want it to convey a single fact. You could also write personalized rejection letters that tell the applicant why they weren’t selected to continue with the hiring process. That could help the applicant in their future job applications. But you don’t do that. I wonder why!

And it’s your pickle to put however much stock you want in the cover letter. That’s not what this is about at all. If I were applying to jobs and I know a significant percent of my cover letters will never be read by a single human on earth, I might be tempted to have an AI write it. If you happen to read it and reject me based on whatever likely-subjective metrics you use to determine it was chatgpt that wrote it, that’s just the way the cookie crumbles. You could also reject me on a million other factors I don’t control. Either way, I saved however many minutes.