Merlin, on two of my older bikes with long reach single pivots I switched to dualies. No other changes to cables, housing or levers. The difference from the singles to the duals was like night and day. It's far more than the rest of the bits in the system. So it all comes down to the leverage ratio.
The things like stiffer arms and better cables and housings helps a little. But the primary advantage of the duals is that there's just more pressure at the pad face for the same amount of lever effort.
If someone made a single pivot that worked with standard levers but had the same leverage ratio as the dualies then this new single pivot design would fee the same. But to do that the arms would be sticking WAY out to the side by the amount that provides that jump from roughly 4:1 to 5.6:1 noted by Drew above. The location of the interarm pivot on the dualies give us the longer effective arm length without the arms sticking out to the side by so far.