Voats problem wasn't engagement. It was literal nazis.
They tried to prop up a thin veneer of legitimacy but at some point they just stopped caring. The front page became blatant "kill all [whatever]" type posts. That's when engagement completely collapsed.
Lemmy has some clearly in bad faith instances which are probably run by nazis. Federation seems to be doing its job of resilience.
Social platforms are under the same effect as voting. Over half population either supports it or is too ill informed. They just want to scroll their feed without thinking too much. Same way voters just vote for the one that sounds alright on the surface. Combined with the fervent supporters makes a majority. It's the minority of voters/users who are opinionated against it.