Unfortunately, they all seem to have died to the “Nazi bar” problem or other issues. A centralised Lemmy variant would likely either lurch to the extreme right, or be rapidly pumped for profit, like Reddit is doing.
One of the advantages of federation is the ability to disconnect groups that won’t mix well. This has already happened with Lemmy. Both extreme right wing (Nazi) and left wing (communist) groups have been de-federated. They are allowed to continue happily, just away from the bulk of users. The fragmented nature also allows for server cost, and workloads to be distributed.
Additionally, Lemmy wasn’t quite ready for the mainstream when Reddit decided to sh#t the bed. I’ve watched it improve impressively, over the last few weeks though. Once it streamlines, it should hopefully provide all of us our fix.