Some of us prefer mailing lists and interacting over email in our preferred client over having to go to various websites and deal with their varying user interfaces.
Except absolutely awful, it's worse than the standard mailman ML and that's saying something.
> you want it to be a forum, but that's not the point or what it is meant to be.
One of the issues being google groups looks a lot like a forum (dedicated web UI, starring, replies to threads, etc…) without any of the good points of a forum.
Basically, for most people google groups is the lowest common denominator of a forum and a mailing list, making it unmitigated garbage.