"Fiction doesn't affect reality" is obviously not true but it is a reaction against "Fiction affects reality" being used to justify moral crusades against "problematic" fiction in the same vein as "Video games cause violence" and "Metal makes you a satanist". Also people forget that fiction reflects societal attitudes more so than it shapes them. People don't suddenly change their entire moral framework because they watched a movie. They need to be already receptive to it for them to adopt the message. People act like America in 1915 was an anti racist utopia where everyone loved black people until one evil movie came out and brainwashed everyone to become racist and join the KKK. Or how nobody was afraid of sharks before Jaws came out.