Another possibility is that parent/offspring incest ideally involves a single parent. Even if the attraction is there, most married men try not to...
Well, no accounting for taste, but one reason could be that father/daughter incest is perceived as more... "molesty" than other forms, so fathers...
Like plan9au said, it's just a legal/policy thing. That's why you see some videos take it a step further with things like "step-cousin" or...