The airframe isn't the main cost driver for these things and cardboard is aerodynamically inefficient. You could blow mould or a million other techniques and get a better, possibly cheaper airframe.

Styrofoam works great for short to medium range glide interceptors that are supposed to be cheaper than the attacking airframe (less weight, less fuel/energy required, less explosive required etc.).

Ukraine seems to be pouring a lot of these right now.