Capt BdS:
There's gotta be a pump somewhere, either on the truck or somewhere underground...
Flight Detent:
Take a look at the diameter of the hoses used for typical overwing refueling and for typical pressure fueling. I think you'll find the pressure hoses are much bigger, as are the openings in the couplings ("nozzles").
If you've ever handled a fire hose, you might know how much of a problem they are to handle. That's why there are typically 2 or 3 firefighters on each hose end. If you had similar pressure and flow rate in an overwing fuel hose, you would risk spills, splashes, and losing control of the hose end.
As far as Young Paul's statement regarding dihedral, it is correct as far as it goes. If the overwing cap is at thr outer/top end of the tank, there may be little unusable space in a tank. However, even with pressure refueling you have to deal with internal float switches and vent valves, so it may or may not be possible to completely fill a tank.