I have zero interest in fucking any of my family members, but the idea of other people fucking theirs can be hot. The wrongness, the taboo nature of it, the danger etc. I think it's a big mistake to assume that just because someone likes incest content they personally want to engage in incest.
It's hard enough for people to make serious money off actual porn these days, I don't see there being any investors willing to put up tens of millions to make a porn game.