This is, for sure, an issue carried over from Reddit, but it’s also a byproduct of another issue we carried over from Reddit: Most posts have a substantive issue.
Obviously not every community has this issue, but so many of them seemingly serve as nothing but news aggregators and do absolutely nothing to promote engagement or discussion. It is no surprise that the quality of comments has dropped when there are entire communities that are just copy-pasted news sludge with no other sense of community or engagement.
When the content on your website feels disposable then people will treat it as such. Lemmy as a whole has this exact same issue as Reddit does, which is not surprising because Lemmy is basically a clone of Reddit. I made a post on Beehaw a while ago about how the instance lacked any sense of community and I’ve seen similar sentiments expressed in other instances here and there. People, such as myself, who expected something better from Lemmy and getting frustrated when we can’t find it. There was a supreme opportunity presented to us when the Reddit migration began, to make new communities and spaces for discussion of a higher quality than Reddit could ever provide but Lemmy completely squandered that. Lemmy sucks - and that’s because it clones so much of Reddit… which also sucks.
Average users do not even remotely care about federated software and/or decentralisation. That is techno-babble to them and their eyes will glaze over if you try to market that to them.
That being said: Mastodon does a removed job at explaining how it works, how to use it, and what its advantages are. The Joinmastodon landing page just assumes you already know how a fair bit about instances work and what federated software is and does a very poor job explaining it. And even then, most users won’t care either way. They just want to click a Join button and be done.