The eternal readiness of mobs to eat your entrails

In WoW, the difference between an unpulled mob and a pulled mob is simply their intent. The former just stands there, the latter want to eat your entrails. The combat capabilities of both are the same, there is no advantage to be gained by a sneak attack unless you have special abilities.

Consider though if mobs by default stand about in a relaxed mode, unbuffed, weapons in the rack, some are asleep, some are not even in full armour. Some might even be absent-mindedly grinding away at some craft-work like cooking or blacksmithing. The side door might be left open. A few guards keep an eye out but that is all. A sneak or rush attack here would be devastating, and would reward the well prepared adventurer group with easier kills and thus more loot for less effort.

Let the guards hear you approach though, let them see you from a mile off, and by the time you get within pulling range you'll find them fully buffed and armed to the teeth, spoiling for a fight. They won't necessarily race to engage though (why would they?). They will however keep an eye on you and if you are wounded or distracted by a tussle with some of the local wild life you just might find yourself on the end of a sneak attack to devastating effect.