"What's the benefit of this to you? What's the benefit of this to the truck driver?"
[Regarding me moving over to the left to let the truck pass me on the right.]
The benefit to him is that he just keeps going. He doesn't have to do anything to deal with me. That's marginally easier for him than switching lanes, passing me, making sure he's safely past me, and then switching back.
The benefit to me is that I feel a little bit safer when I'm not relying on him to perform that maneuver safely.
By the way, most of these vehicles were triple-trailers, a type of LCV ('Longer Combination Vehicle,' or road-train) not legal in 37 states (including your state). I was in Nevada and Utah, at the time.
If a reader is from one of those 37 states, they may have never seen one of these. A photo is
here.
When that large vehicle is near me, doing 80-85 mph, I'm happy if he's just going straight, rather than attempting any maneuvers of any kind (the limit was 75; I was doing about 55). Any needed lane switching is much more easily done by me.
And if I require him to pass me on my left, I'm counting on him to keep track of me, and I'm counting on him to not switch back (into the right lane) too early. When he passes me on my right, it's much easier for him to keep track of me (and I'm certainly having no trouble keeping track of him, no matter what side he's on).
When he's that much bigger than I am (the ratio is roughly 60:1, by weight), when I make things easier for him I end up feeling safer. And they all thanked me.
"A pain? Really? How?"
[Regarding why it's a pain to switch lanes in a big rig.]
When I very deliberately move to the left, when he's about a 1/4 mile behind me, then he knows I'm an alert driver and I'm paying attention to him, and I'm interested in showing him courtesy. (Recall that this is a situation where there is literally miles of visibility, and there is no one else around. So he has been watching me for a while, and he knows what I did is not a random maneuver.)
But let's say I just sit there (in the right lane). When he comes up behind me, with me doing 55, he doesn't know if I'm alert, or awake. He doesn't know if I see him. When he overtakes me on my left, he has to watch me constantly, using his right-hand mirror. What if I decide I don't like being passed? What if I decide to speed up? What if I wander close to him, out of pure carelessness? What if we approach a grade, and suddenly his speed drops, and I carelessly continue to sit in his blind spot?
He also has to make sure to move back at the right time. If he moves back too early, he squashes me. But he also doesn't want to move back too late. Why? Because he has to take into account the possibility that approaching from our rear is a Corvette doing 140, who might be tempted to pass the truck on the right if the truck lingers too long in the left lane.
Everything is much simpler if the one who moves over is me.
"Why for trucks, but not for cars? Why not be consistent?"
I think the various reasons are pretty obvious, but I'll explain one aspect of it a bit more.
The truck is not capable of maneuvering suddenly. That's physically impossible. So at the critical moment, when he's in the right lane and is about to pull alongside me, I can feel sure that he is not going to suddenly change his mind and decide to get behind me instead.
If he were to make that choice (let's say, because he suddenly decided he didn't like the idea of passing me on my right), I would see him start to maneuver, and I would have a relatively large amount of time to react, in whatever way I chose. And I'm much more maneuverable than he is, so it's relatively easy for me to get out of the way, one way or another. And/or downshift and accelerate, if that seems like the best move.
But if the other vehicle is a car, he is physically capable of making that move (to get behind me, instead of passing me on my right) in a very abrupt manner. He could choose to do that. That's a fundamentally much more dangerous situation. Better to just let him pass me in the normal way. Since he's in a car, he can do that very easily and quickly.