Because if they were, people would know explicitly rejected the idea of a Christian nation and explicitly supported secularism and the separation of church and state. They would know that the founders opposed public prayer, believed that the Constitution prohibited legislating Sunday as a day of rest, etc.

It would also learn that many of the founders had pretty disturbing ideas about Black people enslavery.

But people generally don't read the founders, whose readily available. Instead, most people tend to rely on mythology that has grown up over the past couple of centuries.