Answer:
The air around us is permeated with hungry microorganisms that feed on anything that doesn't move fast enough, including us, and the foods we eat. As soon as your food is cooked (and I'm assuming that you have cooked your food thoroughly enough to kill the microorganism that were present beforehand), microorganism in the air will settle on it. Those microorganism will still be on it if you put your ham sandwich in the refrigerator, and since there isn't a vacuum in your refrigerator, there's microorganisms in the air inside your fridge, too. The cold in your fridge slows the growth of those microorganisms, but doesn't stop them entirely. Milk left out on your kitchen table will easily go bad in a day, but milk will last up to a week in your fridge, because the cold slows down the bacteria in milk very well. A freezer extends the spoiling process for much longer. A vacuum freezer, which pumps the air out can extend the storage time of foods dramatically, but it isn't cost effective.