I don't think that is true. There are no fewer than three companies who make shocks that can be valved and rebuilt that will fit on the trucks we use. They each have hundreds or even thousands of dealers who have people in their employ who could recommend a far better shocks than a set of twin tubes with a knob on the side.
The marketing of emulsion shocks for OEM replacement is focused on the "best in class" sort of people. A custom valved reservoir shock is not marketed in the same way or to the same people. Such a product would be marketed to enthusiasts who demand the best.