Pipe threads are for pipes. Pipes flow material at a specific flow rate. That is the critical factor of any given pipe. To call it any other "size" would take away from that fact.
"Its a plumbing thing. You wouldn't understand."
If they called a 1/8 pipe thread 5/16", then dummies like me would reach for the 5/16" straight thread tap instead of the correct tapered tap.
Carl
