When I was younger, I worked in the theater industry. We all knew that the IATSE stage hands were making great wages, but the options we had to join were:

1.) A grueling apprenticeship period in which you would work very long hours for very little pay. I could be wrong but I think this was a several year long process.

2.) You could skip that process if you were vouched for by an existing union member. In practice, this often meant that membership was passed down through families.

I understand that it’s similar for firefighters.

I think that's part of the problem. In germany for example, we have Unions and we have working councils in the companies. And while unions are "just" associations/clubs working councils are are regulated by laws and get elected by the workers of the company. Of course there is some realtionship, but it is not unheard of, that in a working council are members from more than one union (though usually only one has negotiating powers for tariffs, but that's another topic, because usually unions negotiate with associations of companies for the whole sector. And those unions without negotiating power are sometimes not even considered real unions...), and even independent members.