The union lobbies for regulations that require you to apprentice under an existing master in order to enter the trade. The union's masters then purposely limit the number of apprentices they take to limit the supply of license holders. Since the slots are then valuable as a result of artificial scarcity, they get allocated on the basis of nepotism etc. You're not the nephew of anyone currently in the union so you don't get an apprenticeship.

Moreover, getting your own full license takes around a decade, during which time you're expected to work as an underling regardless of whether you already have the capacity to do the job yourself. So to the extent that anyone can get an apprenticeship without blood relations or graft, they expect someone who is willing to be stuck in an entry level position for that long even if they already have significant relevant experience from a related field, and then you get in the queue behind a million high school seniors and never get a slot.