It does accept a username, though.
The first form field prompts for email or username.
The link below the first field works, regardless of whether you supplied the email or the username.
The I forgot my password link also works for either username or email, unless the Admins have enabled the Hide email address taken site setting.
On the OpenAI site, the forgot password form requires an email, but on another site where that setting is disabled, the form allows the entry of the username:
So unless I’m misunderstanding your request, it sounds like we already support what you are requesting.