I wonder how much of it is wishful thinking: A driver pulls up, and thinks "If I do the handbrake/neutral thing, chances are the traffic will move off, just as I've done it, and I'll have to get into gear, handbrake off etc, in a rush." Over time, it becomes a kind of superstition that maybe traffic is about to move off, and so it's not worth doing the handbrake, and that not using the handbrake will somehow make the traffic move...
It's something I admit to being a bit slack about sometimes. In the works vehicle the handbrake is often less effective than the drag from the electric motor anyway!