Not liking something does not stop people from doing it in order to achieve a goal. Goblins may be like people in that respect. You're simplifying goblins to make an argument against their lack of eating requirement.
Im simplifying it a bit because there hereditary attributes and general temperament all point to a individual mentality instead of a pack mentality. You can still be evil, but you don't need to be a pawn of someone else.
Re snpaa... I think you're missing the 'evil nature' part. Yes goblins can be independent but they like doing bad things, so there's a large incentive to band together under a demon or similar such entity to maximise the one thing they do need; death and chaos. As was posted before, there is some exposition of goblins in Threetoe's stories, where you get the impression that gobbos are in equal measure mean and unempathetic because they have no afterlife, thus no future.
There evil nature seems like a cop out reason, since there are plenty of creatures in the game that are evil that don't form huge civilizations. I could see a goblin banding together with others goblins , like for mating, family, or friends and cause panic and dismay with them but not on a large scale like they are now, because forming a civilizations just doesn't benefit them in the long run.
Also If they have no afterlife why waste this life, doing the bidding of some random demon or humanoid? I think that would motivate them to leave the settlement as well.
snpaa: I too find it very displeasing that goblins don't eat or drink. Why stay together, though? They have a little bit of reason to tolerate each other. Imagine if a single goblin attacked your fortress; easily dispatched, even by a mob of untrained civilian dwarves. But an ambush/siege with many goblins is much more difficult to eliminate.
Because they like to cause chaos and misery, they should group together because it lets them produce more chaos and misery in proportion to the number of goblins, and they can all share the chaos and misery with each other.
On another note, the game never says that they don't like inflicting pain and misery upon themselves... in which case, banding together in the miserable goblin settlements would make sense, basically a pain/misery factory.
That's a good point but I think they would change there whole way of attacking, instead of a suicidal 1 goblin attack on a nearly impenetrable fortress they would act more like a glorified boogeyman and kill you while lost and bewildered in a forest.
As I said in an earlier reply I think some would band together (because otherwise how would they keep the species going), but just not on a large scale . Also it doesn't necessarily have to be goblins they team up with, it could be any sort of like minded evil creature out there.