I am currently in the process of installing a piggyback oil drain plug (a bolt that has a bolt through it) and was confused to learn that the inner bolt didn't need a gasket because it was an NPT (National Pipe Thread) bolt. Every other car that I have dealt with has always required a gasket for the oil drain plug.
According to Wikipedia, NPT screw threads and pipe threads "include both tapered and straight thread series for various purposes including rigidity, pressure-tight sealing, or both." If there is a way to design threads to make a proper seal, then why don't oil pans not sure this for their drains and instead require a gasket?
I understand NPT is a U.S. standard, but the concept remains the same. If there is a way to not need a gasket, why would automakers choose not to use this?