As Waldo points out: Cost and durability.

There was a bit of a flap in the media when those guys made the Colt 1911. The whole "OMG, 3d printed firearms being made in basements" and similar BS. The machine used is the size of a small car and cost $5 million. Folks in basements are using standard tools like mills and lathes that have been around for more than a century.

The defence world are getting big into the concept. Imagine an aircraft carrier that could make some of it's own spare parts instead of holding stock or waiting for spares to be flown out. Someone is even looking at re-working some of the standard missiles (Hellfire?) to make the bulk of it 3d printable so that they can be made more quickly (or on demand) in theatre.