You've never seen a non-religious club? Or a political group? Or a volunteer society?

There are plenty of non-religious communities all over the world. Look at any co-op if you want an example.

But the thing is, these non-religious communities tend not to be stable long-term, they're not as well-coordinated, and don't seem to influence people's behavior all that much.

Abstracted from all particulars of a given belief system, religions seem to be a refined, hyperoptimized form of community building.

This isn't true.

Secular society is an example of this. We have common goals that we work towards.

We don't have words for "gather once a week and do xyz" in many context, but that doesn't stop us for doing it.

Join an electronics club and meet up every week for the next 10 years. People do this ALL the time and it isn't an issue.

Some alternative church-like organizations further out on the no-theism-required spectrum (maybe more limited in area affected).

- Ethical Cultures Society

- Unitarian Universalist Church

- Quaker meetings (for non-pastoral Friends, theism is present but not necessary).