Why are all adventure games in slow-mo?
My answer would be: they're not. Certainly not all, and certainly not even most. At least none of the ones I tend to play are (classic LucasArts and DS stuff like Another Code).
Text adventures are pretty quick too. In fact, faster that human walking speed. Just typing "N" I can cover a lot of territory in an instant. Anything from moving out of one room to another, to getting from the bottom to the top of a cliff.