My theory is that it’s because people who are into it must be really into it, but people who aren’t into it are very good at ignoring the fact that it’s titled like that or that there might be one or two throwaway lines implying it (especially when it’s “step”). So there’s an incentive for uploaders to title things like that and for creators to add a little nod in the video towards it (without adding too much incest roleplay) because it draws in a large audience and actively turns away very few.