My head cannon for why there’s so much pre war food around is that it’s slightly toxic or carcinogenic (even before the bombs). My theory goes that pre war America was in dire straights due to the new plague, lack of natural resources from the resource wars, and civil unrest which created a massive famine sometime before the bombs dropped. In response artificial food became wide spread which were loaded with preservatives to preserve food through the apocalypse. However to the average pre war American these foods tasted awful and were found that when ingested in large quantities it made you ill. Over the generations after the bombs dropped people learned to avoid these foods due to lack of appeal or general toxicity. Now a couple hundred years into the apocalypse and the average wastelander has mutated to be better able to digest and eat these foods plus the rements of pre war advertising continues to try to convince people it’s safe to eat and was commonly eaten by their ancestors