Actually that is not the assumption at all. The assumption in both the space shuttle and most other (fixed-wing) aircraft is that the pilot is right handed. However, what you don't expect is that actually most of the really fiddly work happens with the right hand, changing settings, pressing buttons, tuning radio's and writing down notes. The left hand has to be taught to fly but thats actually less demanding that teaching the left hand to do that other stuff. This is even true of aircraft which have yokes rather than side-sticks, as the throttles are controlled by the right hand and during the take-off and landing the right hand is stationed on the throttles.
Interestingly for a helicopter this pattern is reversed, the control inputs required to control a helicopter are so much more demanding that the pilot sits on the right hand side so that he uses his right hand for the primary control and has to either make do with his left or temporarily switch hands to do the other stuff described above.
