Traditionally, they say that because even when their outward trappings are all over popular culture, they know that their core message is difficult to live out in practice, and most of the people who show up aren't doing it.

I think AnimalMuppet is talking about more evangelical churches, whereas you are talking more about what used to be called "mainline" churches.

Before about 300 AD, they weren't the cornerstone of culture. In much of the world, they never were.