Basically because they can -- it's not illegal and you were on a multi-lane road (it sounds like) so they may have felt they could.
So, why? Well truck drivers are paid in a variety of ways (from Wikipedia):
Truck drivers are paid according to many different methods. These include salary, hourly, and a number of methods which can be broadly defined as piece work. Piece work methods may include both a base rate and additional pay. Base rates either compensate drivers by the mile or by the load.
A company driver who makes a number of "less than truckload" (LTL) deliveries via box truck or conventional tractor-trailer may be paid an hourly wage and/or a certain amount per mile, and/or per stop (aka "drop" or "dock bump"), and/or per piece delivered, unloaded, or "tailgated" (moved to the rear of the trailer).
The main advantage of being paid by the mile may be that a driver is rewarded according to measurable accomplishment. The main disadvantage is that what a driver may accomplish is not so directly related to the effort and, perhaps especially, the time required for completion.
And even those salaried or on hourly wages probably get bonuses for completing jobs on time (or, worse, fines for being late). Additionally it may be possible that they're not being paid while empty. All of that adds up to a great pressure to get where your going as fast as (safely) possible. But, there are strict EU rules on hours truckers can work (from the UK government site):
The main points of EU rules on breaks and rest are that you must take:
- at least 11 hours rest every day - you can reduce this to 9 hours rest 3 times between any 2 weekly rest periods
- an unbroken rest period of 45 hours every week - you can reduce this to 24 hours every other week
- a break or breaks totalling at least 45 minutes after no more than 4 hours 30 minutes driving
- your weekly rest after 6 consecutive 24-hour periods of working, starting from the end of the last weekly rest period taken
So, to take a completely made-up to make my point example, assume a driver needs an 11 hour rest period -- they have 13 hours to drive. If they have to drive 1100 km and average 83 km/h then they'll overshoot the 13 hours about 21 km away from their destination and have to start again the next day. But if they average 85 km/h they'll make it inside the window (admittedly with only 3 and a 1/2 minutes to spare). That extra speed could mean they make the delivery on time, they get to sleep in a decent place in town rather than their truck, the truck can be loaded while their sleeping and they can start again with a new job the next day.
Basically, just a little faster can make a difference. And over the career of a driver that can add up. I'd guess they just get used to maxing their distance covered when they can.
Does that apply to that particular driver? Who knows? He may have been in a bad mood, or having some competition with the other drivers or was trying to get home for the birth of his child (OK, now I've just making things up).
Was it a bit crap to pull that move across an exit? Sure. But maybe he didn't know there was an exit, or didn't think anyone wanted to take it. And, as the comments say, there are places where trucks can't overtake -- maybe this was the last chance for a while.
In general truck drivers are don't want to be bad drivers, they want public support for a difficult job (and also for when they strike). Also they're regular road users, they've seen all the bad behavior and don't want to be 'that guy'. Plus there's the common "How's my driving, call: XXX-XXX-XXXX" sign that can land them in trouble.
It depends on when you noticed the exit, and when you realized you weren't going to make it. You might have tried this but I would have been tempted to drop back a little to make sure the truck in front would see you, and turn the indicators (turn signal, blinkers, etc) on to show you want to change lanes. I would think that then, if it was possible and if they noticed, they would try and make space for you to make the exit. I'm not sure if CB radios are still used by truckers, or popular in Europe, but if they are it may only take one to see your signal and arrange something with the others.
It might work, worse case is you're no better off.