The 6000 years thing came from an analysis of the bible tracing back things that happened on known dates up through the begat tree counting backward in time.
People choose the knowledge that makes them happy over knowledge supported by facts all the time. Especially if they're older and the thing they believed was communal general knowledge for most of their lives. The 6000 years thing is more understandable than still believing the world is flat. To prove the world is more than 6000 years old you need to use science that is not plainly observable with the naked eye. To prove the world is round you just need to go around it, which is not that hard to do now with airplanes and lots and lots of people have done it. You'd have to basically think that all the millions of people who have physically traveled around the world are lying to you in a massive conspiracy to trick you into sailing off the edge of the Earth.
People choose the knowledge that makes them happy over knowledge supported by facts all the time. Especially if they're older and the thing they believed was communal general knowledge for most of their lives. The 6000 years thing is more understandable than still believing the world is flat. To prove the world is more than 6000 years old you need to use science that is not plainly observable with the naked eye. To prove the world is round you just need to go around it, which is not that hard to do now with airplanes and lots and lots of people have done it. You'd have to basically think that all the millions of people who have physically traveled around the world are lying to you in a massive conspiracy to trick you into sailing off the edge of the Earth.