This post contains 18+ content. You can view it if you're over 18.
I've always just assumed it's because of like "oh we can't have 'real' (even in a fictional situation) incest because of [laws/advertisers/money processors] so if we take away the genetic component we can get away with it". Like they're simultaneously trying to cater to that kind of pervert and not upset the people who can take their money away.
Like everything else, it's all driven by business decisions, and how people actually feel about things doesn't matter