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.