My reloading manual recommends a charge weight of 55.0 grains of a particular IMR propellant but this much powder won’t fit in my case. I took my wife’s rolling pin and crushed it into much smaller granules. I then weighed out the same charge and it fits fine. Why hadn’t someone thought of this before?

Because when you broke the powder up, you exposed parts of the kernels without a deterrent coating and changed its geometry. This deterrent coating plays a major role in controlling the burning rate of the powder. By altering the powder, you have created an extremely hazardous situation that could result in destroying the rifle and possibly injuring yourself. If you run into a situation in which you are loading a tightly compressed charge (this is perfectly normal with some combinations), you might wish to try using a drop tube to help the powder
settle into the case more compactly. Whatever you do, keep your wife’s rolling pin off the reloading bench, and don’t alter that powder!