"When a measure becomes a target, it ceases to be a good measure" (Goodhart's Law).
Google measures how "good" pages are for a given search query. (It doesn't matter how.) Those who own the pages want those pages listed as high as possible, so the measure of how good their page is becomes a target (for them, not for Google). That's enough to make Goodhart's Law apply to Google rankings.
I want to know what happened. What were the individual things that made sense in isolation, but added up to this steaming turd.