The question was updated to include:
The bearings don't spin freely at all, particularly the drive-side one requires some force to turn. Turning the bearings I feel some distinct "latching", like turning a rotary encoder button on some electronic gadget, or like turning the axle of a dynamo hub (but much weaker). The "latch points" occur in regular intervals (a few degrees), so I don't think dirt is the cause. In contrast, e.g. the axles of the wheels turn completely smoothly inside the hub (but don't spin freely either). I don't think this is normal for the bearings? Can pitting be the problem?
When you rotate the axle or the bearing race, if it feels gritty, that usually indicates wear or contamination. For what the OP now describes, I believe the usual English term is that the bearing feels notchy or that it is binding. That can be caused by the cup or Bb shell being out of round, so that the bearing is compressed at some points in the rotation. In systems with preload, e.g. headsets and cup and cone hubs, if you tightened the system too much, I think it would also feel notchy.
Right now, without being able to examine the whole system, I’d guess that the bearing is the issue. In general, when you press bearing cups into a frame, you need to make sure that they go in straight. You also don't want to hit either of the races too hard. This is why we would use bearing presses and drifts - it is definitely possible to just use a block of wood and a hammer, but you do run an elevated risk of damaging the bearing. It is possible that somehow, at the factory, the bearing wasn't properly installed onto the crank half spindle, and it got damaged that way. This would be atypical of any high-end product, but any industrial process will produce some bad units, and quality control will catch most of them but eventually, something has to slip through the cracks. Your frame could possibly also be out of tolerance (e.g. that side of the BB shell isn't round or parallel to the other). Now that you've removed the crankset, I'd normally expect the bearing to turn normally. However, given the length of use, it's possible that this was the issue and that the bearing is now damaged. Unless you have the appropriate frame tools, checking that the BB is correctly faced and round is probably a shop endeavor.
Given the stated symptoms, I would get a bike store’s help to diagnose the issue, preferably one where the mechanics have at least some Campagnolo experience. If you bought the group online, the store who sold it might have some ideas on what could have gone wrong, although their insight will be limited by not being able to see the system in person. The same is probably true of your country’s Campagnolo distributor.
In response to the OP's second update: I am guessing that the bike manufacturer thinks that one of the cups may have got pressed in a bit crooked. This will load the bearings much more than they were designed to and would probably account for the binding - which will damage the bearings. I'm not sure how common this type of failure is. I would have thought that the frame was the issue. A good bike shop should have the tools to check if the bearing seats are round, and if they are not misaligned (i.e. they have to be concentric and at the same angle, within a very small tolerance).
This YouTube video by Mapdec Cycle Works, a UK bike shop, shows them using a gauge on a second hand bike to show that the bottom bracket had a very slight amount of parallel misalignment - that is, the bearing seats weren't concentric. The difference was very small, but it was just enough to cause premature bearing wear. You could feel the crank shaft binding at some points in the rotation.
They discussed using a specially designed gauge to detect misalignment, and they were discussing taking it into small-scale production. This may imply that not all bike shops have tools like this to detect misalignment. I expect that shops have accurate calipers, so I expect they can tell if the BB shell is under- or over-sized, and possibly if it isn't round.
Last, some clarification on expectations about friction. The OP said above that
after removing the chain from the chainring, when spinning the cranks they spin only for a few seconds (I heard that means the bearings are correctly greased) and I hear a faint, steady "rattling" noise.
It is definitely normal for the cranks to only spin for a few revolutions on modern systems. Bearings have at least two mechanisms which produce drag. There's the drag of the rubber seals against the races. At very low load, e.g. spin cranks by hand, the bearings mainly experience this type of drag. A totally new BB may have more seal drag than a used one. The seals will break in.
When you're pedaling, you're putting well over 100 watts into the cranks. Now, most of the drag comes from the loaded bearings rolling on the races. This amount of drag dwarfs the drag from seals at riding pace. If you search around, you'll see people say you can't spin the cranks to see how much drag there is - this is why.
I haven't installed a crank in a while, so I can't remember for sure, but I don't think noise when spinning the crank is normal. There might be a faint whirring, but I would expect this to go away after some time as the grease gets distributed through the bearing. The OP might have noticed the bearing notchiness while spinning the crank by hand, although it may have been more obvious with the crank out and them rotating each inner race by hand.