I wrote tech docs explaining the jungle of IT systems that we were relying on at a hospital I worked at, and sometimes that included diving into old code. These were usually much longer than READMEs.
Having a README wouldn't have saved this code from needing to be refactored. Nor would it have really changed my opinion of the code. It hasn't been a reliable signal.
The hard part about documentation is keeping it up-to-date and accurate and not filling it with extraneous details and going off on tangents. A lot of the READMEs are written for quick bootstrapping and that isn't going to reflect much on your code quality. I care more about good documentation and that's harder to write than a README and a much better signal.
I don't have time to work on open source but it's clear my experience has been vastly different than yours and I doubt either one of us are going to come up with a peer reviewed reason for either side.
Turn this around and say "this repo has a README! surely it's really good and so is the code" and it makes no sense to give that much credit for something that really isn't impactful beyond the first few days of using something.
