Whether I'm on my bike or running with my stroller, I use my left arm. If I was in a car, I'm not always certain I would see a right hand while I'll always see the left. Also, my security strap for the stroller is on the right arm, so that makes pointing difficult.
Cars have turn signals and on a bicycle someone following you can see the extended arm easier than that funky left arm do all thing. The only reason to continue the left arm do all thing would be you are in fear of the little men in the white coats.