Well for one when someone’s AC is out during the summer, heat is out during the winter, their water doesn’t work, or worse yet is flooding, then that person has a pretty poor negotiating position. Plumbers and HVAC can basically name their price for emergency work.
And the licensing system ensures that everyone has slaved away so long to get into the "I can make the big bucks now" club that they won't compete on price once they're there.
