Just because you can buy the food with SNAP benefits doesn't mean its good for you. Plus plenty of them have additional sources of income and food from family and friends and being disabled.
Also .... those people eat a shit ton. You just haven't watched it enough.
Have breakfast made by a family member, get second breakfast from fastfood (maybe third breakfast), snacks, dinner, 2nd dinner, dessert. I've literally seen a woman eat 3 whole meals from Arby's in one sitting.