I asked my father just this question forty years ago. His answer then was that it was a hangover from early designs, particularly when petrol stations looked like this:
Most drivers pulled in at a pump on their side of the road, which means that most arrivals would have the passenger side of the car adjacent to the pumps. It was all attended service, so the driver didn't have to get out at all.
There is a lot of inertia in physical design. It took a very long time for steering-column stalks on British cars to be set up the same way as other countries' cars with the indicators and lights on the left and wipers on the right — and that change was arguably rather more important. Flash the headlights or squirt the windscreen?
These days, with self-service and different petrol station design, and cars being imported from all over the world, it's less important to cater for the particular instance illustrated. But then again, because cars can pull up at either side of pumps nowadays, there's not really much necessity for each manufacturer to change their own practice.
Illustration: Dan Lockton