Ever seen a mom with her kittens? Or even a dog with her puppies? That scruff of the neck is just extra fat. It's how the mother animal can pick up and carry her babies without them fussing about. When applying pressure to it, it causes them to freeze up.
Considering that you're talking about kittens and puppies, not full-grown dogs and cats, I'd say it's not at all as obvious as you're saying it is. And anyone whose ever tried to lift a dog or cat like that knows that they aren't nearly as docile when a person does at, as they were when their parents did it (that's a major understatement, btw; usually, they try to bite you).
My guess is that you learned this technique from someone, but now you're acting like it's obvious just by looking at the animal's behavior in order to sound like you're teh most clevar.
It doesn't work on either of my cats. They just kinda sit there and start slinking away in a crouch position. If the cat was in any pain it would call out in distress or give some indication of it's discomfort, which I have never seen a cat do when picked up by the scruff.