I personally like incest games quite a lot, only for the taboo aspect. Similar to why I love Netori/Netorare games a lot (Read: cheating). I can't do it in real life, it's not me. But the taboo aspect is very attractive (Read: the shock).
But, this is only because I really have a hard time to get invested into characters by default, in many games. So, the fix comes from the shock value of taboo. As Mr./Mrs. Gunizz said, the psychological aspect of it is super enticing (Would you, wouldn't you? What if someone finds out?).