I guess the government doesn't want the competition.
I read an article about making parts for guns and how making a whole gun isn't out of reach. The article did say that most any gun or part made out of plastic is subject to a lot of pressure, heat etc... and couldn't be expected to last more than a few shots. There is a big difference in printer plastic and poured/formed/vacuumed other methods of shaping. There is a reason guns are made out of steel