Just curious, what's the deal with this concept of mud flaps are for mall crawlers? Aren't folks who actually get off road more inclined to get some? The logic in this thread seems backwards to me, but ok.
If those were on my truck, they would get ripped off immediately (by the ground). I dovetailed my bed for more departure angle after destroying my factory rear bumper in the desert. At full compression, those are practically touching the ground.
Not saying these look bad or aren't functional, but you asked why off-roaders wouldn't want them. It's not that they don't want them, it's that it is impossible to keep them attached.
Not all off-roading is equal. Keep in mind the raptor is build for high speed desert, so this site skews to high speed desert (albeit much less so in recent years). There just isn't a tremendous amount of mud in the desert and I regularly scrape everything on the lower third of the truck. If you were muddin' in the midwest, these might make more sense, but Baja-style running would destroy them.