I think the key difference is that online communities are "cheap"; they're easy to create and easy to destroy. Offline communities are difficult to form and as such more "sticky". A great example is ideological differences. Lefty political groups (no doubt Righty ones have this too but I'm not as familiar with them) constantly reorganize based on perceived ideological bounds. Leftist groups splinter from liberal groups, labor-forward leftist groups split from identity politic leftist groups, and on and on.
A PTA doesn't do that. The folks in the PTA all have the same shared interest in the school their kids attend. They can't just splinter off into another PTA over a perceived difference. This forces the folks on the PTA to work together and makes the organization sticky in a way an online group might not be.
If the activation energy to form and join a community needed it's also really easy to just churn from the community. Moreover when splitting is this easy it prompts the creation of hyper-specific communities which lead to things like radicalization and dehumanization of the other (look at the acrimony between leftist identity-politic progressives and center-left liberals on the internet right now for example.)
A PTA doesn't do that. The folks in the PTA all have the same shared interest in the school their kids attend. They can't just splinter off into another PTA over a perceived difference. This forces the folks on the PTA to work together and makes the organization sticky in a way an online group might not be.
If the activation energy to form and join a community needed it's also really easy to just churn from the community. Moreover when splitting is this easy it prompts the creation of hyper-specific communities which lead to things like radicalization and dehumanization of the other (look at the acrimony between leftist identity-politic progressives and center-left liberals on the internet right now for example.)