Because

A) Solving Captchas isn’t protecting from abuse/spam anymore. People in countries with cheap labour costs are being paid (or forced) to solve these for spam networks. And nowadays, LLMs can solve them almost better than any human. Manual approval is completely infeasible once you have a somewhat larger following.

Tying comments to some form of account is at least somewhat of a hurdle for spammers.

and

B) Some people want to keep ownership of their data. As long as the comment is tied to my account, I can easily find, edit or even delete it. Try that with some comment you made on some obscure blog 5 years ago; which address you don’t remember and with an email address you no longer have.

Why does commenting on a blog post need any kind of account?

Reply notifications, commenting history.

capcha

I don't know about you, but I hate those, and wouldn't want people to have to go through that, as I wouldn't want to have to go through that.

manual approval

That's a lot more work, and it delays the conversation when replying to another visitor.

My guess would be trolls and/or bullying

I would love to be wrong indeed. Would you have some links ? When I asked on the Ghost subreddit, multiple people said Ghost v6 doesn't do Fediverse commenting.

They've been working on this for a year and a half.