Because Most People
Yes, both left and right-side driving arise from the same fact although for different reasons.
In the days of highwaymen and robbers, you wanted to stay to the left so that if you needed to defend yourself your weapon and stronger right arm were facing the oncoming traveller. This was the general practice throughout Europe. Napoleon and others changed to the right on the continent, but island Britain continues on the left to this day.2
By the time Europeans came to America, travellers no longer used swords for self-defense. Here, a wagon driver with a multi-horse team preferred to sit on the left so he could use his right arm to whip the horses. For him was easier to see oncoming traffic if he drove on the right. Right-side driving was first formalized on the Philadelphia and Lancaster Turnpike in 1792 and state-wide by statute in New York in 1804.4
- Illustration from the 1903 edition of The Story of King Arthur and His Knights, Wikipedia.
- Why do some countries drive on the left and others on the right? World Standards web site.
- Detail from Conestoga Wagon by Newbold Hough Trotter, 1883. Photo by Ad Meskens, Wikipedia.
- Richard F. Weingroff, On The Right Side of the Road, Federal Highway Administration web site.