Well, it's unfortunate, but it's not baffling. It's quite understandable that there's no single sign-on for a system like this.

If you want to have both decentralization and a shared "identity" across all of the decentralized servers then you're probably going to need something like a blockchain to accomplish that. But people are already complaining about how complicated the Fediverse is, so adding a blockchain into the backbone will likely be challenging to pull off. Not to mention the knee-jerk reaction a lot of people have to the word "blockchain" or "cryptocurrency" regardless of what the actual practical application of it might be.