Some states allow passing on the right, but even in those states it is discouraged.
Passing on the left only applies to freeways/highways/interstates, whichever term you use. Exits and entrances, also called on ramps and off ramps, to the highway are typically on the right so merging traffic is also on the right. Merging traffic is at slower speeds, so keeping slower traffic on the right is safer and logical. Passing traffic is faster traffic. As traffic is entering the highway the car entering and the cars in the right lane carry the responsibility of cars merging on safely and so if another moves over to the right where cars are merging it adds another car to have to deal with and a car changing lanes can easily be missed by the car trying to merge onto the highway. Much like you cannot change lanes within a few hundred feet of an intersection or near an intersection, no one should be changing lanes into the right lane near an entrance to a highway as cars merge on.
A lot of old highways have weave patterns for exiting and entering the highway, which means cars exiting are merging over to the exit lane while people coming on are in that same lane eventually moving over to the left onto the right lane of the highway. In older cities the merge space is very short, made in a time when speeds were not so fast and highways were not so crowded. Some old highways you have to basically yield rather than match speed and merge.
In summary, merging is the real deal on freeways and you can’t merge well with traffic if you are trying to get up to speed and the cars in the lane you are merging with are going so fast it is impossible to reach that speed in the distance alloted before the merge lane ends.
Lastly, the left lane is the fast lane, so it is logical that it would be the passing lane. It is safer for there to be general rules about the speed of each lane on large highways because a car driving 70 that comes up on a car going 50 is unsafe. Especially, if the road has some curves and the driver might not be able to see several hundred feet ahead at all times.
Note: highway in this case is the same as an interstate type road not a rural highway.