See Ella’s previous posts from earlier on in this forum thread that gives you the reason why…
Now that we have the login API, you can create your custom login page, and add a link from the signup custom page to the login page (eg., "already a member? click here to login ).
In addition, in the member signup settings panel, under the “which option shows first” section you need to select " New member signup ".
This way the default login page will never be triggered by an external application / restricted page / login bar, and only the custom signup page will be opened with a link to the custom login page.