Because when the outside of an item containing flour is exposed to high heat, it will dry out and cook faster than the rest of it, which essentially is like sealing in the moisture. This makes it more difficult for the internal moisture to escape, keeping it inside, thus this is why bread is baked so long......it takes more time for the moisture to escape the dough in order for the internal ingredients to be baked.
And, depending on what kind of bread you are making, you can do different things to the exterior of the bread/dough to get different results in the crust.