just with kbin.social (and of course other badly moderated instances) specifically.
Yes, there’s clearly a moderation problem, but it’s not just kbin.social.
To rephrase the question: why is it that practically all the insufficiently moderated sites are kbin instances?
kbin currently only supports one admin per instance
An overwhelmed admin could partially explain why a lot of spam comes from a particular instance. But it wouldn’t explain why most of the small instances generating spam have chosen kbin instead of lemmy.