I've seen games based on very recognizable IP (intelectual properties) with an open Patreon, which has greatly surprised me. Not that I intend to follow their steps (my cute little game about an abusive, corpse-loving vampire family will never be accepted there) and monetize it, but it made me worry about them a bit.
It depend of a lot of things.
Basically speaking, it's the opposition between the right for parody (in the game author case), and the right to protect your image (for the owner of the IP).
Therefore, in one hand there's this question : Is your game effectively a parody or is it just using the same visual, visual codes, universe, whatever ?
And on the other hand, there's this one : Have the product a negative impact for the image of the "product" ?
If it's not a parody, you're domed, same if it have an effectively negative impact for the image of the "product". And there isn't many impact more negative than a porn game.
Now, for the question by itself, well... I think that the answer is really simple : The devs aren't aware of the risk.
I mean, Patreon clearly ban incest since a long time now, reason why there's so many "landlady games" and so many incest patch. Yet, there's few games on Patreons (that I'll not name for evident reasons) that are directly, clearly and effectively incest games, without any artifice to make them looks like something else. Theirs authors don't know, or don't care, and the same apply for most of the games using protected content.
And another question: could be a campaign of a tabletop RPG in videogame form be considered as an abuse of the IP if you provide it for free?
RPG's rules are intellectual property like any other. Therefore, yes it's an abuse, unless they are under the open game license, or any similar license. But here it's still another mater than the one above, because the world, names and rules are relatively shared between RPGs.
By example, you can't be sued because you simulate a D20 for the attributes check, because your characters have an Armor Class or Health Points. In the exact same way that you can't be sued because your text use a sentence that happen to be in another text. Here it's the percent of similitude which will determine if it's your system, inspired by [whatever RPG], or if you used directly the rules of [the said RPG].
But for the background, it fall back to what is said on top of my comment. You can't have the same towns, the same characters and all. But you can have towns or characters that will make players think about your inspiration. By example, a desert planet, full of sand, on a Sci-Fi universe, will make them think about Tatooine or Dune, depending of what live on this planet. But as long as it's named differently and that the people living in it are just "look alike", normally you're safe.