The obvious reason is Forum has been around for years and if it would be the missing link we wouldn't have this discussion.
I really love planets, its problem as I see it, is of participation, I find that interesting planets, are normally from huge projects, where you have at least one high quality blog a week, and if they do not, they look stale. My feeling of planets is that the threshold for participation is high, you actually have to have a blog to participate. How many people have a blog on FreeCAD? My hunch is that this prerequisite of a blog is a bit to high for FreeCAD at the moment If not some additional content comes from low barrier of entry participation solution, this could increase the planet release frequency and might give it a vitality boost.
One solution could be a new topic called "planet entries" in the forum, where blog style posts could be posted by people that do not wish to maintain their own blog. This could of cause be moderated in some way. Posts from the threads such as "User Showcases", "Release Announcements" etc. could also have semi/automated reportings to the planet
This way, or with automated filtering by "up" votes as described earlier in my last post, could be described as the missing link for medium and small projects such as FreeCAD. And it could all be driven from a central forum.
Not only would you give life to the planet in this way, you would also direct people reading the posts to the forum we all love and rely on. This is something that other planets miss, they redirect to individual blog sites and dilutes participation. Replying to a blog post is nowhere near the value of a reply here on the forum.
It could very well be that no one is doing something like this today, and that there is no such solution available out there, for our forum software. But I do not think that we should be afraid of imagining stuff, if we do not, it will never exist.