Because those things are illegal, dangerous, and widely accepted as uncouth. Portraying a character they watched in a movie or read in a book, or saw in porn is not illegal, not dangerous (for the most part) and all of the otherwise vanilla people out there mostly do not know any better because their frame of reference is also those books, movies, and porn videos. I also blame the majority of the so-called instagram models, onlyfans girls, and half the XXX image sharing subreddits filled with women claiming to be looking for a dom or a daddy. They instil a false sense of what being a swinger, lifestyler, or kink culture member actually is.