> when did attending church become unusual in America?
It’s not that common in elite liberal circles, especially in the NE corridor and California.
Note that some people in this group do go to church, but it’s just not widely advertised and is typically not a central part of their lives (e.g.(compared to some places where going to church for some sort of event 2-3 times on Sunday and 1-2 times during the week is a thing).
Also note that every Ivy League school was founded around a church and the church-based seminary/school. The same is true of many of the elite feeder prep schools. Note that there is a church prominently in the middle of all of the campuses.
That said, rightly or wrongly, churches are associated with anti-science folks who are anathema to most academics and academic types.