With similar weapons why does an AH64 Apache require a pilot and gunner when the A10 Warthog only needs a pilot?
Mainly it's because the A10's pilot just has to point his aircraft at the target from a certain height while in forward flight, whereas the helicopter, being able to hover, needs to be able to engage a target while not pointed directly at it, while the pilot is busy flying a machine that takes a lot more attention than a fixed wing aircraft.  

Especially when considering the Apache's original tactical function, which was to hover below the tree tops, pop up and engage a Soviet tank, and drop out of sight again.  As the pilot you wouldn't want to be managing the weapon systems AND flying the machine and avoiding obstacles.