Because one of the security functions of a browser extension (as demonstrated by this very issue) is to ensure that a password for one site only goes to the sam

Because one of the security functions of a browser extension (as demonstrated by this very issue) is to ensure that a password for one site only goes to the same site, and not to a convincing-to-a-human phishing site. A browser extension that you can rely on to fill in your password is therefore better for your security than one where you get in the habit of deciding you know better than it and end up copying and pasting passwords and making judgments yourself.