Now you can argue that the central joystick should be oriented fore-aft/left-right on axis with the aircraft, but now you are inducing a very painful twist to either arm, and the stick movement is no longer in-line with any of the arms natural axis.
Except that your analysis is not borne out in reality...
I have flown several aircraft with central control sticks from Cubs and similar light airplanes to fighter jets. I have also flown the AH-1W Cobra helicopter with a right-side control stick that moved (yes, actually MOVED) in the normal fore-aft and left-right axes. NONE of those were even remotely "painful." All of them fell naturally to hand.
While a ergonomically shaped and positioned stick grip is better than a straight stick like the Cub's, even the straight stick poses no problems for translating the arm/hand's 45 deg orientation to the 90 deg orientation of the control movement. The reason is that the stick is anchored, not free-floating, and the arm/wrist/hand readily adapts to the control movements. The brain has no problem translating.