1) In places where the unit of bargaining is sectoral, not firm or plant or job or agency, strong unions don't confer a competitive disadvantage by bargaining well. Structurally, unionization in the US stands in tension with employer success in a way it does not elsewhere;

2) In part because there is no implacable misalignment, unions and employers are frenemies: Yes, unions and employers fight over respective shares of the pie (in money, benefit, conditions including safety, etc)… But they are also able to recognize a shared commonality of interest.

Both unions and employers want their industry to thrive, and can find mutual advantage in efficiency gains, a bigger pie to split.

This kind of reasoning is short-circuited in a US context, because union activity is pitted against the competitive efficiency of particular employers from the get-go.

Some alignment still happens — unions and employers might both support a tariff. But in basic, fundamental, ways, US unions are set up to overlook employer efficiency, because the alternative is to concede everything.

If each union agrees it mustn't bargain too hard for fear of kneecapping an employer relative to some competitor, what results is no bargaining power at all, a race to the bottom across unionized shops.