Why dual pivot brakes, not longer brake arms?
The single pivot sidepull design requires a fairly wide pad gap at the rim in order for centering to not be too finicky. It's essentially the widest of all common rim brake designs. This is due to the spring design's two sliding contacts, as Jobst Brandt writes about here. The contacts are hard to keep at the exact same level of friction, and any disparity takes the brake out of center. You need a wide gap to compensate, at least for a normal person not fiddling with their brakes all the time.
You can't just add longer actuation arms to make the same design more powerful because it's still the same human hand at the lever end. If you want the pads to be able to touch the rim, then longer actuation arms with the same pad gap would require either shorter lower arms, which you can't do because there's a tire there, or a lever that pulls more cable. You can make the lever pull more cable, as in a v-brake lever, but note that v-brakes have a much smaller pad gap. That's why they can have a lever that generates much less mechanical advantage. Do the same thing with a hypothetical long-armed single pivot and you get a brake with very low power.
In other words, all brake designs are bottlenecked by the input force. You can't then have one that's also bottlenecked by pad gap and have it compete on power with ones that aren't.
What amount of pad gap should be designed in to the brake to overcome the centering issues is subject to both opinion and potential design advancement. You may even disagree with the premise that this is a design limitation, and that's fine; every real advancement in component design does. If you made a single pivot brake that could happily run with the pad gap of a dual pivot, then yes you could increase the arm length to get the mechanical advantage of a dual pivot. There's presumably some way of doing it with graphene.