I think there are multiple reasons for this.
-
Lots of new users (me included) who don’t know the ins and outs of Lemmy are joining. Some may not find the right communities for their posts (e.g. if they don’t exist on their instance, but on another) and post it to a somewhat related community on theirs.
-
There are many communities (often one per instance) that deal with the same topic, therefore generating similar posts in reaction to an event relating to that topic. While they might consolidate somewhat over time, I see it more as a feature than a bug of the fediverse. You could pick the one(s) you like most and unsubscribe from the rest.
-
Until there is a critical mass of users niche content will get little to no engagement while big current topics get almost all of it. These posts will be up there when sorting by top, hot or active, I’d imagine. Maybe you could solve that by sorting by newest.
Reddit screwing over app devs also potentially means app devs free to use their skills for Lemmy/Kbin.