Before milling, the husks are intact and serve to protect the grains.
After milling, the grain "guts" are more exposed to moisture.
Like mentioned above, they will last a little while after milling, but the clock is ticking. Use them ASAP.
If you want them to last longer, I would suggest not milling, keeping them dark and dry.
I like to compare the grains to coffee: Once your coffee is ground, keep it airtight and in the dark and cool. It will spoil quickly if not. Beans keep much longer. If you compare the exposed surface area of whole beans (or grains) to the ground coffee (or milled grains) you'll quickly see why.