in Pittsburgh, part of the reason is that there are some exit ramps off the left lane. People who are afraid to change lanes get in the left lane and stay there for miles so they don't have to merge to get to the ramp
And we have weaves (2 roads merging and then splitting) that are much too short, and on ramps that have stop signs as you enter the highway. Sometimes the right becomes an exit only as well. Our roads are a mess, but I love them. Driving almost anywhere else feels easy.