Because it's a load of bull,All you have to do is smell it,if it does not smell bad it's not (according to some food scientist) I believe it to be true.
try that with chicken the date stamps on most foods will be good for a few extra days, they have to date it earlier than it needs to be, and of course it depends if the food is kept in the conditions its supposed to be,either stored in a cool place or refrigerated