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Why would that be? Let me count a few ways.
1. Some of them don't have a UU church in commuting distance. Above is an old map, but it shows huge swaths of our nation (anything not pink or purple) where there is no UU church in the county. Not much has changed in 12 years. Some people who are out of range of a brick and mortar church belong to the Church of the Larger Fellowship, and more may now that this organization has really beefed up its on-line resources, but most apparently don't.
2. Some live in range of a UU church but don't belong because they can't find a place for themselves theologically, or they don't like the minister or the leadership group, or they have become discouraged by church politics or burned out by the incessant demands of lay leadership. We are a denomination of small congregations...and small congregations are hard, hard work. There are very few communities outside of the East Coast between Washington DC and Southern Maine which offer any real choice of UU congregations.
3. They may live in range of a UU church but be busy with other things in their life right now. Many college students are near UU groups but don't join up, for instance. Their lives are rich and interesting and busy on campus. It would take a huge effort of outreach and support to get them interested in belonging to a UU church. (I get this. My son belongs to this group)
4. They may have grown up as UU's and not continued to belong to a UU church, although, if asked, they would say they were UU's because they generally agree with what they were taught as children. That is to say, they may be among the between 80 and 90% of children of our church schools who don't join UU churches ever again in their lives. If the goal of our RE programs of the past 50 years had been to innoculate children against church, we'd consider ourselves quite successful. Ouch. My two siblings belong to this group.
5. They may be kind of interested in being a UU but when they visit they discover nobody like them. Nobody under 40. Nobody who didn't graduate from college. Nobody who is not white. Nobody who is not Anglo. They look around and see that in this congregation, they'd be by themself. So...they stay by themself.
6. Then there are the ones who don't want to be asked for money but don't mind taking advantage of the fact that some people will give for them. But this is, in the end, a pretty small category.
Put all these folks together, and it is easy for me to believe that 3 out of 4 people who think of themselves as UU's would not actually belong to a congregation.
How about you? Can you add other reasons UU's might not belong to a UU church?