The interior of a firearm with a firearm control group, firing pin, etc., is kind of detailed. On top of that, to manufacture it with more moldable soft plastic would be very hard because the resultant product wouldn't be substantial enough to withstand the pressures of a bullet's expelled energy.

You might have been able to manufacture some hard-plastic extruded gun in the past, but as with this one, it wouldn't have been great, and it would have cost a lot in the way of equipment. The only real novelty here is that it's doable from home on a relatively inexpensive device.