Take a longer look at the new POF, it IS a side folder. The piston and spring are over the barrel.
With the AR system, part of the inherent accuracy is the straight line action. No out of axis propulsion of the bolt, straight back action, light and compact. Not a problem for the average shooter, military or civilian. Shortening the barrel and making the stock adjustable doesn't make it a better shooter, just a little more convenient wearing armor and getting out of wheeled vehicles in urban areas. A side folder doesn't offer more than that, either.
Portability vs. tactical accuracy usually means the professionals wind up using a fixed stock - even on machine guns. It's really rare to "need" a folding stock as it has such a limited application.
It helps to market a rifle with it, as POF obviously took time and effort to have one made up special, when a fixed stock on a (useless) buffer tube would have done as much. How exciting would that be demonstrating the lack of a buffer and relocated action spring? Not so much.