This is just a theory about bad left-lane drivers in the U.S. - where I live - but here goes:

Many drivers in the U.S. are uncomfortable driving and using their mirrors, most likely due to how easy it is to get a license and not having to undergo the extensive driver training as is required in Germany.

If you have a 3 lane highway, the middle lane is scary because you have other drivers on both sides of you. The right lane is scary because on-ramps merge with your lane. The left lane is the most comfortable lane because you are buffered on your left side by the median and only have the lane on your right to worry about.

Hence, you switch to the left lane, drive slowly, and stay put. Not to mention - annoy every decent driver stuck behind you.