Driving annoyances

Take an interstate highway, two lanes in each direction, with a 65 mph speed limit.

In the right lane, a semi travelling 65 mph.

In the left lane, passing the semi in the right lane, a semi travelling 65.0000001 mph.

Time drags as the semi on the left doesn't accelerate, and the semi on the right doesn't decelerate.

I understand that many semis have governors that cap their speed, but why doesn't the semi being passed slow down?