> not changing my overall feeling that credit ratings are junk science

Then you have been misinformed. Rated bonds by and large fail at the rating predicted by the big three [1].

[1] https://www.ft.com/content/d97e1e86-1e90-4c5c-b972-052d43a36...

I know absolutely nothing about this, but it does strike me it might be self-fulfilling? Companies that have access to more investment tend to win more often, and thus become 'investment grade', while junk-rated firms fail because they don't have the runway?