It's pretty standard to attack sleeping opponents in RPGs as well. Nothing strange about it. It's also why military camps must have lookouts in RL. What comes to the case in this show, was it the goblins who claimed that big city from humans? If it was, then they certainly should be expecting a retribution. And even if they weren't the ones, they are still living in a former human settlement. I reckon the goblins are depicted like that for the exact reason of making the adventurers' life look harder in this show. Especially with the slow and gruesome death of the first goblin. Nothing is glorious in this show (except maybe Shihoru's body).