1. Passing on the right, i.e. passenger side, means being in the driver's blind spot longer.
2. On- and off-ramps are almost always on the right, as are police making traffic stops, ambulances, cars pulled over on the shoulder, sidewalks, pedestrians, cross streets, etc.
3. Slower traffic is supposed to be in the right lane (because of #2), so a car accelerating to pass the car in front of it, suddenly in a lane of slow traffic, is a safety hazard to the slower drivers, regardless of #2.
I'm writing from the US, so for me left lane = driver's side.
The right lanes are generally explicitly intended to be slower lanes. Making them optionally faster lanes can be dangerously disruptive. As well as, when the rightmost lane is used to pass, difficult for new vehicles to merge from side roads.
These laws are made so people know what to expect for safe driving. When we're all being chauffeured in autonomous vehicles it'll be safe to rethink them.
