One of the biggest concerns in any apocalyptic situation, be it nuclear, zombies, or environmental disaster, is trying to find enough to eat. Military style rations may not be particularly appetizing, but they won't take up a lot of room in your pack and can keep you alive for a lot longer than just about anything else. There's a reason that soldiers have these in the middle of wild jungles. All of the running around and fighting you're going to be doing is going to cause your body to go through a ton of protein and carbohydrates, both of which are essential parts of a person's energy. Although carbs are technically not required for the body to function, they tend to be a much more efficient energy source that relying purely on protein and fats. The more you can carb and protein up, the longer you can go, and the folks who make military rations definitely have this in mind when designing these rations.