Because "flat" fenders (you're talking about ones that still follow the shape of the wheel longitudinally but aren't 'domed', right??) are not as stable, they rattle far more. Perhaps worse than that, the rear fender, unless reinforced well, will slap against the rear tire when riding on rough surfaces/potholed streets.
Eventually (I had this happen), a little groove was worn in the aluminum fender from repeatedly slapping the tire. Where did all that metal go??? It may be coincidence, but my rear tire blew out. This wasn't a cheapo stock tire either (kevlar belted Panaracer Pasela Tourguard). It was enough to get me to put a "proper" fender on the rear.
I think "flat" fenders are meant to by stylistically appealing but far inferior to traditional fenders.