Why do so many people think America is a Christian nation founded by Christians?

Some of our most prominent founding fathers like Thomas Jefferson, John Adams, Ben Franklin, George Washington, Thomas Paine, etc. were not Christians. They were most likely all Deists. The Treaty of Tripoli clearly stated "As the Government of the United States of America is not, in any sense, founded on the Christian religion". Even prominent historical figures like Magellan, Napoleon, Galileo, and Da Vinci refuted Christianity. I don't get it!!