That's what surprises me, especially because both games I mentioned are already rated 18, and for streamers, there's usually an option to disable such content. So, it's not even a matter of additional work, and it wouldn't negatively impact sales since mature content can be disabled for streaming.
The games are already rated 18. But including graphic depictions of sex can cause them to instead be rated AO, which is a higher classification and is outright banned in certain countries and digital storefronts.
As far as not "Negatively impacting sales" that's just flat out incorrect. Sales are impacted by anything that can cause a game to receive a higher classification or restriction in availablility. Some countries heavily restrict certain types of content (If you spend enough time on the internet you'll probably come across Japan's overly zealous censorship of sex leading to the whole "Pixelated genitalia" phenomena in pornographic material)
So just because a streamer can activate streaming mode and then stream the game on a streaming website, doesn't mean that the game won't be restricted in its distribution or that parents would be less likely to buy the game for their teenaged children.
But if you include the option to be completely naked, you should either make use of it or invest the energy you would spend on it elsewhere.
To be fair, it doesn't take much resources to actually implement. Especially since you're going to code/model a baseline "Naked" model anyway, there's little difference between the textures including some form of underwear or genitalia.
What would take effort, would be making scenes to utilize genitalia. Since you'd have to choreograph the scene for each race due to size differences (Not to mention the differences that would occur in regards to male and female PC's) as well as additional animation for the actual interaction with the genitalia (Unless you wanna go low effort and just have the penis clip through the model)
