To expand a bit, until dual pivot side pull brakes were popular there was no reason to have direct mount brakes - the bolt going through both arms also held the brake on. There were two aluminum arms on each brake, one for the left, one for the right.
With the advent of dual pivot, i.e. a pivot on each side of the brake, there was a third aluminum arm, the one that holds the two posts.
In this stock picture (from CC in this case) the third bar is visible in the front brake. The rear brake is the old style side pull brake. The dual pivot brake uses the center bolt as one pivot and a second offset pivot as the second. The latter gives more leverage than the center bolt so ideally you'd want two offset pivots, aka like an old style center pull brake.
With a direct mount brake you get the better pivot placement and no extra material on the brake. I figure the frame/fork would need a bit of reinforcement to resist flexing, just like when V-brakes were originally introduced - if you retrofitted V-brakes on a lighter regular-cantilever brake frame you could see the seat stays flexing out as you applied the brakes. Once frames got reinforced the V-brakes worked fine, but on older non-V-brake designed frames the V-brakes did not improve braking.