I think it's a human tendency to want open areas to veiw. A hunter placed at the edge of woods/field will nearly always spend more time looking at the field, even though (generally) he would see more deer in the woods.

Obviously, the further away you can see them coming, the better your chances are of success since you can get most of your moves in while they're still too far away to realize you're in the same state.

As already pointed out, shots in the thick stuff can come and go very quickly, so you definitely have to be more alert which, incidentally, means more mental fatigue. It's all part of the price you pay for a crack at those big dudes that hang out there.