The point of incest plots, as with most "forbidden love" plots whether gay or incest or whatever, is not generally "fetish" as such. Sure, sometimes, in your more "ecchi" oriented manga. But mostly, the point is that fiction usually requires conflict to hold attention. In a romance, love that's forbidden/taboo has a built in conflict that can either drive or at a minimum add a layer to the plot. In the earlier courtship phases the relative rarity of the type of attraction heightens the likelihood that attraction will not be shared, while its taboo nature means that revealing the attraction comes with heightened risks, higher stakes. In the later, mutual love, stages the taboo nature of the relationship creates conflict between the lovers and the rest of the world--family, friends, society. They will have problems from hiding or problems from others resisting their relationship.
So overall, it's just easier to make a forbidden love into a good story.