Same story as SEO, I presume. Google surely understood their ranking criteria deeply, yet couldn't exactly win that fight.
There weren't that many with high ranking. I could filter out most of them in like a week of working normally and just right clicking them with the blacklist extension in Firefox.
Nowadays it is probably harder though when Google have broken the internet culture.
But there seemed to be some time lag for a SEO spam sites to climb. Removing the ones that make it would mess up the economics at the time. Like the Stack Overflow mirror sites or like spam phone number pages with no other information but the number.
And I guess plenty of search queries would have no moderation. But I mean I spent like 10 minutes. You could have 10 FTE doing that.