This is due to decades of conditioning by the Left in education, politics, entertainment and news media.

When every college professor, teen drama and news program you watch growing up tells you that Christians and conservatives are evil, bigoted, backwards people and you live in a bubble where you don't meet and get to know Christians or conservatives IRL, then that's ultimately what you'll end up believing.

The average Democrat on the street probably doesn't pay much attention to politics and is probably oblivious to the conditioning that they've experienced but most of the Democrat thought-leaders probably know it isn't true, but since it's a useful lie, they continue to perpetrate it.