I think it's more that that every cause wants to be become a cult or a habit. The one thing causes don't want is to became reliable ways of achieving their stated goal.
Part of it is that people need social networks, but at least until the net, it was hard to make those networks happen just because people needed them. Either there had to be a cause, or the networks happened by geographical default.
When the goal of a organization is no longer feasible, the organization may look for another goal rather than dissolve. I'm not cynical about disease-fighting organizations which don't go away just because the disease has been abolished. It's simply too hard to build substantial organizations.
Thinking about your original post some more, I'm wondering if perhaps fun groups tend to involve flow on the part of many participants?
If that's an element of it, it would seem an inherently delicate arrangement, because flow requires people to be challenged, but not too challenged, and practice in those circumstances frequently produces growth in skills and a need to increase the level of challenge. If people grow at different speeds they might need different challenges and no longer find association productive. Or you might have a cohort with an init...
I suggest that there are default patterns for social groups, and they could be viewed as high entropy-- what you'd expect without knowing more than that there was a social group of a certain size, possibly with some modifications for tech level and status.
For example, I think that authoritarianism is the default for government-- "we're in charge because we're in charge, and it would be dangerous for anyone who tries to change that". Totalitarianism is lower entropy-- it's surprising for the people in charge to have an ideology which requires them to make drastic changes.
The recent Elitist Jerks: A Well-kept Garden describes an effort to fight one sort of entropy (the repetition of the same questions and answers) which resulted in another sort of entropy (an excessively stable and eventually fragile core group).
Maintaining fun is another challenge in the keeping things alive category. Pleasant is relatively easy. Fun (which I'd say requires novelty) is harder, and I'm interested in comments on what it takes to keep the fun going.
There's a theory that life exists as chaos on the border between order and randomness-- I find this plausible, and it's a different angle for looking at the Friendliness problem. How can a system be built which continues to permit (or even encourage) interesting sorts of change, without permitting change so drastic that we as we are now wouldn't recognize the outcome as still related to us?