They've seen what they can do, or they've seen what they might do, but for the most part not how, and how hard it is, to do the same outside the class. What they lack is a communicative tool. We don't challenge that assumption, and encourage our students to form such groups. But few do. There are some purely online writers groups, and they are popular.
People prefer virtual contact to physical isolation.
But virtual contact with persons who only ever exist as text is also very difficult to maintain or trust. Some such virtual communities are built around known writers working as mentors in the first mode we described earlier. Some students find mentors who can teach through the medium and whose comments are truly helpful. Others do not. In either case participation in such a community means starting over.