It’s just easier for everybody to implement. Password manager opens https://<some-website>/.well-known/change-password in the user’s browser, it gets redirected to the actual page where password change form is located. You could make the password manager look it up in a link tree and then open a correct page, yes, but...

> I guess the more flexible it is, the harder adoption becomes

Yeah. If there is one account management related URL that password managers care about, it’s the change password page. You don’t really need to change email on your account that often, but it is probably a good idea to rotate your password once in a while. So I guess it’s a good idea to make it as easy as possible to adopt – which means just a single URL redirecting to another.

> If the website implements it.

That’s a good catch, though. I guess right now password managers would still have to make a “preflight” request just to see if /.well-known/change-password is implemented before showing it to the user. (But that can go away if most websites adopt it.)