That option isn't always available, at least in the US. Unless you live in a right-to-work state, you may be forced to join the union as a condition of employment.

Somehow this is seen as "more progressive."

There is a conflicting tension.

Freeriding

If a union negotiates better conditions at a workplace, who should be subject to them? Everybody, of course IMO

But what of people who never paid union dues?

There is no nice tidy solution to that tension, only messy ones that impinge on a freedom somewhere

It is worth unionising, voluntarily