Think of it as dimensionality reduction—many people claim to be detail-oriented, but mistakes on a resume are an easy way for reviewers to verify that claim.
Spelling issues are inexcusable given how easy they are to catch (I hear Microsoft Office has a spell check feature), but grammar issues beyond that should largely be overlooked.
It wasn't about skills or experience or culture fit, but about getting the hiring done in a sane amount of time.
But they're not easy ways. There's absolutely no correlation between resume quality and candidate performance, and they're just idiosyncrasies we develop over time. We trick ourselves into believing these signals are important, when they're not.
I don't think that's entirely true. The resume is a product of a potential hire, and it gives off the first impression of their organizational and writing skills. While it shouldn't be the _only_ deciding factor, that first impression is an important signal for a hiring manager who has to filter potentially hundreds of candidates.
That's like saying a first date is in indication of a couples future. It's a bit silly. Sure, if it's absolutely chock-full of errors that could be a sign, but one or two are the kind of things you're supposed to ignore.
> that first impression is an important signal for a hiring manager
Only because people have been trained to judge others based on petty things, so people have responded in kind. Not because it's actually a valuable effort, but to give people one less reason to throw it in the "No" pile.