I can't think of too many food products that are making people fat that can be called fresh and high-quality.

Fruit and vegetables come to mind.

That's a good question. I think it has to do with a lot of food that are not calorie dense and more likely to satiate you simply taste better fresh and not so great past a certain age. On the flip side, foods that tends to taste good for a long time also tend to be calorie dense and some studies suggest aren't likely to trigger the signals for satiation.