Because we're in a period of rapid user migration and not everyone is aware of what communities already exist, or have a different idea about how they should be structured. After a couple months of people shifting between servers everyone will get settled and we'll see which communities will survive vs which ones will be determined to be redundant.
It's not just limited to Lemmy and kbin. Technically any Fediverse-compatible site can interact with them (ie. Mastodon, Pixelfed, Calckey, etc). It's just nicest to do it from Lemmy or kbin because they are the most Reddit-like.