I really don't understand how you reconcile that with unions pushing for federal and state wide wage / benefit increases that cover non-union employees as well. You need a structure with which to organize labor power. Call it whatever you want—whether it's a union or a soviet or a syndicate it's the same idea.

Voting is literally just the bare minimum bottom of the barrel scraping. You have to also agitate for things to be on the ballot, and there is plenty of pressure to be applied outside of the purely electoral lens. A union gives people a direct benefit for organizing while trying to improve conditions for workers in general.