I think it is because some of our foods contain minute traces of carbohydrate but they are insignificant. ( eggs )
also -a lot of our vitamins and nutrients come from vegetables which in the main are very low carb but do contain enough to be counted.
cabbage , celery , kale , spinach , broccoli , cauliflower are good examples of veg that contain such low amounts of carbohydrate that you will be full eating any of these long before you have eaten 10 grams of carbohydrate in 1 meal.
perhaps you could give some examples of the 0 carb foods you refer to ??