The dominant non-commercial social organ for most people in their local community is their church. Atheists are by definition cut off from this.
There are exceptions of course, Unitarian churches etc, but I would posit that most atheists don't find a comparable social grouping.
Once you joined a church (it didn't really matter which one), you gained a lot of social connections that turned into business connections. I don't know if there was any statistical truth to that, but it was definitely accepted in my family's social circle as a rule.
Care to put forth any evidence that this might possibly be the case?