Online communities self-select and self-censor. A student with a contrary, incorrect, or controversial viewpoint in a classroom has a voice and can spark discussion given they operate with best intentions and have a decent instructor. They can be wrong one day, come in the next day and continue to speak. Such a user in an online community gets downvoted, muted, banned, ignored, hidden, or any of those things that keep the chamber full of its own echoes.

In theory these communities can exist online. In reality, the vast majority of online communities are not open to being a classroom setting.