Got guns? Check. Other suvivors? Check. Food and water? Check. A place to hold out? Check. Now let's zoom ahead about a month or so. Is the food you haven't touched still good? If they are processed stuff like Twinkies, potato chips, or Cheese Doodles, you're probably still good. However, for stuff that really gets your body going like bread, milk, fruits and other stuff like that, that stuff won't last beyond a week and will be rotten by then. Now imagine this happening in the entire country, limiting the food supply. Only junk food will be left and you don't know if the store down the block hasn't been raided already and your current supply is gonna run out eventually. To make matters worse, there's a good chance some places will have already riotied, destroying power to buildings. Now if a store has no power, then it can't keep certain foods frozen or cool, making them spoil early. This is made worse if the zombies happen to attack during the summer, cause the hot and humid air spoils food pretty easily.
In short, fresh foods that sit too long, plus no power to keep them fresh, plus exposure to the air will greatly reduce the food supply as they spoil. You might prepare the best you can, but this is something I never see people talk about.