Two reasons.
First because of progression, once your getting fucked up the ass by strangers, how can the game continue to escalate? There is only a little bit to be had in roughness or fetish stuff without changing your audience to people wanting mostly that. So like how clothing very slowly gets more feminine and sluttier, sex starts as a bit of foreplay. Players who want to see mostly inside the bedroom content aren't going to play a life sim, and players who want a life sim will quickly get bored if you throw a lot of heavy porn early, and don't have a carrot to dangle ahead for them to unlock.
Secondly because of the scenes actually available as media files to use. If you want impactful scenes not a random grab bag of clips, you need to limit yourself to only a handful of actors to represent each in game character. Seeing as the public content to scrape is only the final cut of films not all the working angles ect, devs are forced to write scenes around a wider range of topics.You can't write 20 normal romantic sex scenes between the same people if there are only 2-3 videos shot together between those actors.