People merging onto the highway are supposed to reach highway speed before attempting to merge, but they very often do not do that.
And they're supposed to to find a gap, but there may not be one or other drivers may deliberately try to deny them one.
The merging lane may be very short.
The merging car may start to change lanes even if it's not safe to do so.
Congestion may require more deliberately courteous behavior.
The right lane often has many trucks clustered together and rote exercise of the keep-right rule will require an excessive amount of lane changing.
And on a limited access highway, lane changing is the most dangerous routine maneuver so one should be thoughtful of balancing lane changing with expediency.
And the idea that a driver has no responsibility to accommodate another merging driver, if applied strictly, also means that one cannot always leave the right lane at will, because driver in the left lane are not permitting. So you change speed a lot, which is rough on fuel efficiency and precludes cruise control if it's adaptive.
Being religious about keep-right is kind of miserable on a congested highway, especially one with a differential of speed speed limits depending on vehicle type. The people most steadfast about traffic laws get punished at the pleasure of people who flout them, which doesn't seem correct.