Ordinary food gets spoiled by being exposed to air (oxygen) and by being consumed by microbes (bacteria, mold, fungi, etc).
Canned food is sealed in airtight cans, and then heated to kill all microbes in the can. So there is no air inside the can, and no microbes. Unfortunately this also cooks the food inside, which is why there is no such thing as canned salad greens or canned medium-rare steak.
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