The second problem is that ammunition is heavy and carrying an infinite amount of it increases your need for food and water in a world where your biggest threat

The second problem is that ammunition is heavy and carrying an infinite amount of it increases your need for food and water in a world where your biggest threats are cholera a dysentery. And unless you want to find yourself among jittery poorly-prepared armed people, you can't just show up to Cabellas to steal more ammo.