I’m not arguing that the United States was founded as a Christian nation. It wasn’t.
Lack of regular church attendance is not indicative of people not believing in Christianity.
The colonies and the early United States were exceptionally rural societies; neighbors might be miles apart. Rural villages were often a crossroads with 1-2 buildings. The locals were likely poor. The money and/or time to build churches likely didn’t exist; and, why build a church when few people could reach it and return home in one day’s travel.
Most roads in the rural US weren’t paved until the 20th century. Those roads were mudpits for much of the winter and after most heavy rains. Getting to-and-from church wasn’t guaranteed; and, cows needed milking 7 days a week, twice a day.
Some of those people who didn’t regularly attend church practiced their faith at home.
That said, even if a vast majority of American citizens regularly attended church and were openly Christians (they weren’t), the Founding Fathers were not all Christians and they did not create a Christian nation. The Constitution wasn’t created by a majority of Americans; it was created by a tiny minority.