Interesting observations. I agree, most of the incest fantasies--especially intergenerational ones--would be total turn-offs in real life to the people who enjoy them in imagination, literature, or video.
But there's another angle, more apparent in the porn video content, where most of the offerings involve acceptance and affirmation of attraction between stepsiblings or stepparents and teenage stepchildren. There, I suspect some of the high demand comes from actual arousal experienced by people in blended families.
There are some videos that play up the taboo angle, but not nearly as many as those where "It's okay because we aren't actually related" is incorporated into the dialog.