What I don't undertand, though, is why unionizing is so granular in the US.
State laws differ, which gives different unions different powers in different places, requires different structures in different places, and offers different levels of influence. But it's also important to note that the American experiment with socialism lasted from Huey Long's election as governor of Louisiana in 1928 until Reagan's election as president in 1980, with the bulk of American unionization coinciding with the New Deal and WWII.
American unions also took a real blow from the unfolding of British labor history in the '70s. Fundamentally? The UK is a rump state whose future was over at the Blitz but it took 30 years for it to become apparent that two rounds of lend/lease and the slow-motion rebellion of the Commonwealth were depriving the British Empire of the resources it had commanded for 200 years. Things came to a head in '78 and the UK elected Thatcher, who effectively destroyed the UK as anything other than a generator of reality television and quirky humor. When air traffic controllers struck in 1981, Reagan took his cue from Thatcher and fired them all.
See that abrupt rise in public unions in the '80s? Followed by a discontinuity? yeah.
So on the one hand, you've got an embattled proletariat facing existential neoliberal warfare. But on the other hand, union employment, as in "I work for the union", is an extremely cushy grift. You make "union scale" for pushing paper around and showing up to other people's workplaces. Unions protect their own hierarchy first and foremost, and fuck the membership. I'm a 10-year fully-vested member of IATSE and my union is a clown show. Those who can, do. Those who can't, organize.
Unions are also dirty AF. There's a reason the Teamsters are associated with organized crime, they're Mafia. Best way to be somebody in the Union is to be related to somebody in the Union. Thus, unions grow indolent and abusive; I had a project manager ask some of my contractors in NY once if they could maybe "hurry it up" because overages were costing us about $30k a day on a simple retail install. "Lady," the foreman said, "We got two speeds and you don't want to see the other one." This is after an envelope with $50k in cash was dropped into a mailbox in Brooklyn; it was an actual line item in the budget.
Add to that the exciting adventure of "right to work" states, a Republican distinction that says "just because you have a union here doesn't mean you have to be in the union to do this." Washington is a right-to-work state. California is not. If I, a sound mixer, decide to double a violin in a score recorded in California, the violinist must legally be paid double. If I, a sound mixer, decide to double a violin in a score recorded in Washington, the violinist doesn't even get notified. Combine that structure with national-scale unions and you get exciting shenanigans like the one where my deeply venal, deeply corrupt union president took the national union to the supreme court because the Louisiana local bitched about jurisdictional issues. Note that Jim Osburn is a deeply incompetent man whose girlfriend made $140k a year to supervise an unpaid film school intern who answered the phone three days a week and who in the Year of Our Lord 2008 walked into a friend's broadcast truck to ask where the Nagra went. Go look up a Nagra. I'll wait.
Unions are a net good, but power corrupts. In the United States, unions have largely become organs who protect mid-century dynasties at all costs in an environment where their extinction is a perennial goal for half the electorate. They have to survive in a patchwork of laws through legal, quasi-legal and illegal means and as a consequence, they are a subject of contention. And again, I say this as a fully vested member of IATSE whose grandfather was regional president of the AFL. Unions have been good to me. They have also been shit. I lost $20k because my union forgot to tell me "oh by the way you have to buy a ticket from Los Angeles in order to make that rate." I have friends who just got their healthcare taken away because SAG decided that they couldn't afford to keep their promises. All of Hollywood was out of work for four months in 2008 because a producer and his son-in-law concocted a scheme to break a million tiny production companies through force majeure.
Unions are a bloodsport in the USA.