> These are parts that take very little stress and can be relatively easily printed and used to hold together all the other parts that actually hold the stress of firing the bullet.

A lot of the polymer guns (1911, AR15) need to be reinforced with metal at certain places for any kind of reliablity. A Glock doesn't need to be, because the material was invented by the designer of the gun and the gun was intended to be a polymer frame from the start.