As a matter of fact, very few parts are identical. Mainly headsets, brake clamps, levers, rockrings and... well that's about it. Boosters have different stiffness and weight, some cranks are more rigid, some are lighter. Most bars have different geometry as well. A year ago you could also buy Adamant, Czar etc. hubs, but those have been discontinued. Frankly speaking, stocking all these items is a nightmare. As for the reason for all this, there are people who will want to change their headset and brake clamps after getting a new frame. It's all just a matter of maximizing income.