I notice that a lot of UAVs out there have generous amounts of dihedral, to the point that they look like the old fashioned "free flight" models of yesteryear.
But UAVs are not free flight models, nor are they full sized aircraft or models of same. UAVs all have an obsessive - compulsive autopilot that keeps an eye on the flight in a way that no human pilot ever could (for very long). In fact, there is a design out there for a plank style flying wing that depends on the blistering speed of high quality servos to close the control loop - i.e. it is aerodynamically unstable.
UAVs are also not models of a full sized aircraft, so don't have to slavishly follow full sized aircraft conventions.
So why aren't we building wings that are essentially not terribly stable and leaving some of the work to the autopilot? It would make them much easier to build and a flat centre section would be massively stronger and more rigid than mucking around with little bits of carbon rod and copper tubes at the right dihedral angle.