It's not just an American thing.
I sat down a few years ago and had a real good chat with a local priest, just kind of shooting the breeze. At one point we talked a bit about their aging congregation and the struggles of mainline churches to appeal to younger people. The subject of Mega Churches came up and one of the things they told me was that those kinds of churches actually have a really high turnover rate. As in, 90% of congregates stick around for a year at best, and most check out well before then. I don't know how to look up if that's actually tree or not, but talking to quite a few Christian friends of mine over the years, it seems pretty believable. The size of those churches add an energy to everything, almost like going to a concert, but unless you find a clique or something to fall into, there's not a real sense of community or personal empowerment. Coffee shop and bar churches on the other hand, are like the other end of the spectrum.
Me? I'm not Christian, but sometimes I like going to a church, sitting down, and hear someone preach for a while. I've found I've been pretty comfortable with both mainline churches and coffee shop churches. Both tend to be pretty welcoming in the sense they're like "Hey, new face! Welcome!" and I'm like "Thanks! I'm just kind of passing through and just hear for some coffee and a little sermon, if that's alright." It almost always is.