Why do trucks pass each other on the highway?

I think as RoflcoptrException says, a small speed gain can make the difference.

The drivers can have a huge distance to travel, and (in the EU) the rules on allowed time before breaks is strict and enforced via tachograph etc and an extra 40-50km a day can be the difference between making the truck stop and stopping in the middle of nowhere.

As an example, I awoke one day with a big articulated lorry parked in a siding outside my house.

The French driver was pleasant (we gave him real coffee in a flask), he was delivering the next day to a company in the village we lived in (this was a Sunday), but was unwilling to move, even though a better place existed a couple of minutes up the road.

The reason, he had hit is maximum time when he arrived overnight, and although he could move the truck if necessary, he would have to restart his break time for the tachograph, meaning a delay before returning home when he unloaded the next day.

Luckily(!) for me, the siding wasn't really up to the weight of the truck (wheels started to sink into the mud), so later on (mid-afternoon) he had to make the move, and was still sitting outside the destination company the next afternoon, despite unloading at 8am.

You should also remember that truckers are very sensitive to fuel economy, and practice many similar techniques to hypermilers. This includes using their brakes as little as possible, and maintaining a constant speed as much as possible. They also are trying to save time, as others have noted.

What this does mean is, when they are able to, they will do silly things like pass another truck, even if slowly. Mike of Trucking Truth explains exactly this: they pass each other to try and make up the time, but sometimes between one thing and another it doesn't work out that they are able to complete the pass in a reasonable amount of time. The slower truck won't slow down because that costs momentum which costs fuel; the faster truck isn't as able to deal with an incline or similar; and so they parallel for a length of road. The faster truck of course wants to pass not only to get to [destination] on time, but also to avoid slowing down himself in many cases.

In the US in major cities we tend to have regulations prohibiting trucks from driving in the leftmost lane (or lanes) when there are multiple lanes, to help avoid this.

One more reason: trucks are much more sensitive to slopes. While the cruise speed of two trucks may be almost the same when it's flat, it may be quite different going uphill. However, uphill passing can be difficult: sometimes it's specifically disallowed, acceleration is harder, there may not be a good opening when you need it and visibility tends to be less. It really slows you down if you could make a hill at 80 km/h but you are stuck behind a fully loaded truck than can only do 60 km/h. Even if you eventually can pass, just getting back to 80 km/h will take some time (and a lot of extra fuel). Hence the preference to pass when it it's flat, even if there isn't a significant speed gain at the moment.