I cannot comment generally on why it seems such a hard thing to master, as I used to be under impression that it should be the basic skill and that everyone is capable of doing it with a bit of practice.
Perhaps the reason I cannot do it is because I associate it with porn movies, and apart from the obvious sex act the rest is just plain terrible acting (or am I watching wrong kind of porn).
Also if acting is required on WG's part then I must be a terrible punt, as I never feel a need to act in any sense, I do and behave the same as I do in my personal sex life, and talking dirty is not a part of it.
Sorry can't offer any better explanation, as this is the subject that I often wonder about .