Many propane fittings have pipe threads, so you kind of start out with that standard. Also, some forge designs are based on iron pipe (for example, a piece of 1/8" schedule 80threaded inside to 1/4-28 to accept a MIG nozzle). Many forges have very few fittings between a propane hose and the tube that holds the orifice, so it wouldn't really be worth the trouble switch from threaded to sweated and back. And have you seen the price of copper lately?
