It's also unfair because plumbers deal with buildings decades old - some even a century. They may have extremely old systems and knowledge of that is important.
Where I live in Los Angeles we recently acquired the USS Iowa, a WWII battleship built in the 1930's. One day I was there, going to the bathroom, and one of the maintenance workers happened to be there and we started chatting. Turns out he was a former SeaBee who was there to retrofit the very old saltwater-based plumbing system to connect to the civilian freshwater system so people could use the head in a normal way. A very non-trivial and specific skillset. If you have ever visited a museum ship you can be sure they had people who knew this stuff.
