Each state has its own laws with regard to the legality of semis driving in the left lane.
However, in most states, semi-trucks can only legally drive in the left lane when they are:
passing another vehicle,
exiting to the left, or
making a left-hand turn.
When commercial trucks primarily stay in the passing lane, it can lead to dangerous truck accidents, severe injuries, and sometimes death.
Note that on a multilane highway, the designated lanes where large trucks and motor vehicles can operate are typically expanded.
Three-lane roadways restrict trucks to the right lane, but they can pass in the middle lane.
On a four-lane highway or more, truck tractors pulling semitrailers can operate in the two right-hand lanes.