The idea was to remove the bars across the back of the brake. The best comparison I can think of is center pull brakes vs roller cam brakes.
The extra parts I believe are for extra clearance so they're a red herring. Consider that there'd normally be a very rigid aluminum or maybe carbon? piece holding the two mounting posts. You save that with the direct mount brakes.
No matter what the complicated mechanism above the brake, the direct mount frame saves the bar that would hold the two mounting posts.
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