I was one of those humans, and it's more that the algorithms were initially designed under the assumption that we would keep doing that work for free, forever,

I was one of those humans, and it's more that the algorithms were initially designed under the assumption that we would keep doing that work for free, forever, but once the communities we were doing it for were destroyed, why bother? Also there was a lot of pushback and poo-pooing the human element; lots of well-regarded (at the time) people saying that there was no future in human-curated content, and I know that I (as a young person) believed them. Look at the cool stuff they were making; they have to be right! They're smart adults, they would know, right? It just stopped being fun to curate.