Some of these trades do have apprenticeship program's in their unions and at least in my city in the licensing requirements. To be a journeyman electrician you are first an apprentice for 4 or five years, and at least in union shops your pay reflects your (theoretical) ability.

So a fifth year apprentice make something like 80% of a journeyman as they should be capable of 80% of his tasks; while a first year apprentice pulls in less than half as he likely knows little of the trade.