Holymoly, this question could be solved easily which some simple answer but it still blow up into huge discussion
First of all , would you fuck your female family members if given a chance (mom/sister/daughter) If yes then you are IRL sick unless you being influence by external factor include culture ethnic , family tradition (what?) , etc
Not to say is IRL sick , if you like incest ONLY in game then no problem that is just taboo kink you like.
Problem is we live in modern age now where incest is like 2 to 5% occuring around us as a impulse sex due to curiosity or puberty went wrong or wrong common sense teaching when young. When milk is spilt , you might not able to change the fact you did fuck your female family member but it could be changed NOW if you decide to man the fuck up and discipline your cock to keep it in your pant,seriously. When the baby start to pop then there are some problem now you gonna handle on your own now with court and doctor and other government in charge with degradation view from society, your friends ,your neighbour ,your coworker then some stranger busybody ass who just randomly pry shet on you.At that time , i guess you don't deal with it then just run away literally and figuratively
Back then , people decide to do incest because well noble like to keep their ranking top and keep their blood "pure", that why they commit to it. Hierarchy system belong to noble well until the rise of the commoner. Noble who decide to do "it" with commoner will be shunned by society back then . Commoner when can't find mate will "mate" with their opposite gender under desperate situation well not exactly but still you know what i trying to explain .Then there were no advanced science back then , if anyone found doing so called science will be labeled as witchery and burned on stake, in thise case Biology. That why there were no improvement for science until 1799. because of war and shet, plague and other life sustaining situation. No science equal no evidence after all.
Once the inbred theory is realized and finalized, the rate of incest is decrease alright.
So why incest is still existed now? well sexual kink is a thing and people mind somehow complex.
The things is why we does play incest games because of mutiple factors due to (Shit, why my sister hot or damn my mother look young)(No dad around , sexual frustration go high.)Story (Sex abuse, no daddy don't)
The look and the young impulse of age does create so called incest. Family sex abuse is another case now.
Take from common incest VN, why we player attracted to the girl in game ,well she look nice and doesn't look like grandma. That is our first or second thought but this came to our mind later (She is my sister, mom , daughter {wtf daddy})
Even we knew this is taboo and wrong, we still disobey it and fuck the shit out of her , just like secretly eat more than 3 candy even though mom said you can only eat one.
This is still pleasure of guiltiness disguise by our dopamine. Which can be applied to NTR and other fuckup kinky ass.
Now that this answer solve portion for the question
Another answer would be the creation of incest which is the developer. Now why developer want to do incest, that would be each to their own . For me , it would be opening a route to be conquered by MC so that player could enjoy my content more and spend their time in MY game .Their money in MY game. Nothing more . Why player fap because attractiveness of character, probably high artstyle of drawing by the artist of game. Like KIMO . High caliber artist can create highly sexual art if given chance , that would be art sexual (wut?). The main things is that would be your body react to it , your boner but doesn't mean you want to fuck her/him. That would be your sexual pheromones releasing on it own , nothing else (So many anime girls could get avoid getting raped if they just ignored it and kick the ball of rapist , girl "boner" suck).So you try to ease your boner in front of you, but does that make you a incest? No! You just try to get rid of horiness, that all. Seriously though, let liking incest be a fetish that you like, not something you would do in real life. You saw pretty girl and you like it, That it.
Well, essay done i guess.