Incest is writing shorthand for forbidden love.
It is immediately recognized as taboo for most settings.
The protagonist is expected to be near to the person they desire, even when pushed away.
The protagonist is expected to have a history and emotional connection to the person they desire.
The protagonist and love interest start with something to lose, creating conflict and consequence.
That is a lot of freight to put into one word.
It makes it easier to write, like using any other archetype.
You could absolutely write the same thing with your boss/student/minister/neighbor or even just the cute bartender. It just takes you more time to build the connections and consequences before you move into the romance portion of the plot. This is part of the appeal of infidelity as well; you want to fuck someone you shouldn't, they are tangential to your life, you both feel the connection, and you both have something to lose.
Not to say it isn't a fetish, it absolutely is. If you get to go through the whole game with "that's my kink" on your mind, then it's a lot more satisfying than romancing a flight attendant. Unless you're in to flight attendants. Or your mom is a flight attendant. Of course, if you fetishize redheads and the entire game is smoking hot redheads, you're just as lucky.