Whole wheat berries last a good long while if stored in an airtight container. But ground wheat berries oxidize and spoil really fast, unless you filter out the bran and the germ, which I very much doubt ancient people were bothering to do. And lean bread only lasts a couple days before either molding or going rock hard. Basically what I’m saying is that grinding up flour and making it into bread is actually a pretty shit method of preservation, we’d be better off just eating a porridge made from whole wheat berries, and I bet bread stuck around just because it’s friggin delicious.