There's also less complexity for a centralised system, since you don't have a big confusing mess having to learn which server you want to sign up with, how that impacts what you see, and how you connect with other servers.

It's one of the downsides of Lemmy, since people get completely boggled over their heads, and either jump to the biggest Instances that they can find (assuming that the servers are basically completely separate), or give up on it entirely because it's too confusing when you just want something simple and straightforward.