Bad assumption... I have both.

I would never choose flatbars for a ride of greater than 1-2 hours. I don't have bar ends because they get hooked on trees on singletrack. I do get elbow, wrist and hand fatigue from flat bars after an hour which doesn't happen with drop bars because I have more positions.

I can also get a lot lower on drop bars, despite having a XC position on my flatbars. (Same reach to center of bars on both bikes.) This helps a lot in Kansas wind.