You know, I had a fridge logic moment today at work and came to conclusion that part of the reason for that might be that D&D has this, hmmm, reputation for lack of a better work of a "less roleplaying" system, as opposed to the RP-heavy ones like, for example, anything by White Wolf. What I mean is look at the video you linked - the party there does roleplay their characters, obviously, but to me at least they approach it in a more light-hearted way. They focus on having fun first, "getting into character" second so to say. Which might lead to a different chemistry than other way around.
But maybe it's just me...