Because in case of login required forums /login page is their homepage which is there to give anonymous users more information about the community they want access to, and how to get access.
In case of public forums since the forum is open, anonymous user directly sees what the community is all about via /latest or /categories page, so /login page doesn’t makes sense and we show login modal instead.
Also, in case of login required forums, some Admins want to further customize their /login page by adding background image, customized text, etc. So it doesn’t make sense to force login modal there. Some examples of such communities:
https://circleforum.squarespace.com/login
https://community.pleasecreate.com/login
I hope this makes it clear.