I've always said it's because they don't know what they're listening for.

I'm of the opinion wild turkeys don't often do the little "pppfffffftttt" (spit) thing, and I think that's what people are listening for. (Or maybe I'm just not listening for it, either way I just don't hear spitting as often as I do drumming)

I've had turkeys drumming in the woods before, and more than an audible sound, it sounded like the pressure dropped....like a wave from high to low, then back high (usually followed by the tapering off boom, boom, booom, boom, boom, booom drumming sound). I tell people, "hum in your throat" low enough that you can feel it in your ears....you're listening for what sounds like someone with a really deep voice humming to themselves.