The reason it is so simple for righties to reel a spinning reel with their left hand is because of the diameter/circumference of excursion being much larger with a spinning reel having a longer crank. A baitcaster's shorter crank, forces your hand into a much smaller circle, like beating scrambled eggs, especially if you are trying to reel at warp speed.

Very few people can scramble eggs with a perfect tight circular motion using their non-dominant hand, it's just too fine a movement. I don't care how much you practice reeling with your non-dominant hand, it will NEVER crank a baitcaster as fast as your dominant hand.

So when you have the need for speed, righties should crank their baitcasters with their right hand, or go for a left-crank spinning reel with a high gear ratio.