I agree with you that you can actually make useful things with a 3D printer, but pipe fittings are tough. A pipe fitting is tapered and the outward (hoop) stress will split the layers apart easily. I have had some success in wrapping the fitting with thread and then epoxy coating the thread reinforcement in place. For straight threads in ABS printed parts, I have used heat set threaded inserts with great success, but those are not watertight. There are some watertight, straight thread types that use an oring to seal. I have not tried those.