I agree, but it's not unique to Phoenix. In the PHP world the otherwise excellent Symfony framework is only starting to add generation of authentication/authorization, and email confirmation and password reset remains the developer's responsibility.
I see two reasons for this.
1. Frameworks see themselves as building blocks, not RAD systems. When the core framework developers are (mostly) working on APIs they don't see the need for approval/password reset. After all "you'll issue the user with a JWT so who cares about the rest (shrug)"
2. Flexibility to auth with anything. Apparently not bundling a system makes it more "flexible".
I believe (1) to (a) be shortsighted and (b) we need RAD frameworks, of higher-level flexible components that can be bolted together.
I believe (2) is wrong, and bundling standard auth flows does not reduce flexibility: you implement your own if you need something else.