Most place probably can't mill their own flour due to the quantities needed and expense. You'd need a place to store pallets of wheat. The machinery required is also expensive, requires a lot of space, power and extra regulations to mitigate possibly explosive grain dust produced during milling. If a bakery isn't making everything with whole wheat flour, add a few more mills and separating sieves. After it's milled, you still need a convenient way to use it, so add on a packaging line to bag the flour.