You’re missing an important part in what marriage does: Formal representation for each other, may it be law or health. That means you have someone can make decisions for you if you’re not able to (or get information about your where abouts). If both expect to stay together some time it simplifies a lot of things.

And if it turns out you don’t like each other anymore, there is always the option to get divorced.

Unions are worker associations, aren’t they? So it’s up for the members of the union to decide what they want. My, non US, union has different tracks for management and experts (so no problem to move up or change tracks), has a solid base pay but the company is encouraged to pay more than that. For the real rockstar employees there is there option to get paid well above union, then their rules can be mostly ignored (because you’re earning a shitton of money as compensation)

Oh, and for a part of hard and dangerous work they’re currently fighting for 4 days at full pay.

Not getting unionized is like going to war alone because you don’t like the uniforms colour…