In theory the high wing as an aerodynamic advantage in that the suction surface (the more critical one) is unbroken by the fuselage, meaning the flow should stay attached over the whole surface. It's similar to why the swan neck supports are all the rage for rear wings on race cars these days.
But as was said, there are pros and cons to every design. Visibility is more important than outright efficiency for a fighter since they've got plenty of power and fuel to burn.
It is interesting to me that sailplanes seem to be all mid-wings.
