Now I don't ride MTBs as much as many of you, but I do enough miles on the road in winter on my hard tail, and now I'm riding in the woods 2x weekly (combined trail/road ride of about 2-2.5 hours each time out). I got my first hard tail in 1992 or so. I have NEVER had a flat riding a knobby tire (I ride 2.1" tires). I would imagine they're pretty hard to puncture, primarily because the knobby surface largely eliminates a major cause, which is sharp debris sticking to the tire surface and working its way through the rubber. It's much harder for items to stick to the actual tire surface since the knobbies are doing the rolling. MTB tires are also thicker than 700c road race clinchers.
Is that typical? I'm still carrying a spare tube and I'm prepared, but it never seems to happen.