This is a valid question, which could also be asked of Alien. It’s as simple as some people like to be scared, whether to explore personal feelings on a specific type of fear or purely to be scared. For some players, that a game addresses a fear they rarely explore is an enormous bonus.

Your confusion is understandable. Games that directly address the same themes of sexual violence as Alien are a minuscule niche inside an already small niche. But I can tell you as a horror GM that even a whiff of an exotic, earnestly held fear, as long as the player is willing to engage, cuts deeper than hours of classic slasher horror. It doesn’t have to go as far as even Alien, just a little taboo horror as seasoning, but even that needs consent.

D&D is very poorly weighted for hardcore horror. I don't think I mentioned D&D in my original comment but I could be wrong. Other ttrpgs are way better. Shadows of Esterun, Call of Cthulhu, SLA Industries and Dread would be better options if you want to dip your toes in.

It's way more common for aspects of sexual violence to be sort of more alluded to in the past tense and almost NEVER happen to a PC unless the player themselves makes it an aspect of their character that happened in the past. It's damn near never something graphically described at the table in real time.

Inclusion realistically often looks like small references made by the NPC living victims of serial killers being held captive who speak a little about the horrors they've experienced but trail off before they get graphic... A bit of somebody's backstory or allusions to the weird monster that kidnaps and beheads it's victims is doing it because it of weird reproductive purpose... but it is a rare table that actually will not call a FULL stop to play if someone starts full on beat for beat trying to describe a detailed rape scene in progress or a monster basically doing weird reproductive stuff to an NPC in present tense much less a player.

Most of the time with something like that you employ "veils" where something as an idea is introduced as a factor to make something more horrible but you don't really describe it in detail. You let the abstraction make it tolerable.

I personally might consider use of extremely mild themes of reproductive horror in a game but I personally draw the line at targeted sexual assault being any part of that. I neither want to risk triggering somebody's PTSD or anybody's weird anime porn related kink by accident so it's not something I would personally run. I am not personally triggered by their inclusion but with straight up rape it's easy for the way things play out to be in poor taste and only a few GMs I have played with actually used the themes for anything that felt thematically poignant and not just trashy. Most of the time the risks just outweigh the rewards by magnitudes and in a safety focused culture that shit flies like a chunk of lead.

Mind you old school tables were pretty brutal, even your average D and ;D campaign not billing itself as a horror might have had a rape situation thrown in as window dressing for a sacked town or female prisoners in a camp. I remember a couple of DMs I used to play with really thought nothing about chucking it in just to make stuff feel gritty and "realistic". The culture of tabletop has moved into a much kinder place in the past decade for which I am personally quite grateful. What was once the domain of horror gaming safety techniques have been adopted by regular players now.