Because not every rule is up for vote. Ex: Majority of Americans want weed federally legal. America is a democracy. Why is that not the law? It's not up for vote.

There's two ways to unionize: roll your own, or UaaS. Rolling your own requires a lot of work, time in committee meetings, and has a risk that you'll overlook common issues or flub negotiations due to inexperience. But there's a lot of flexibility, and it can be done on the cheap. A lot of people pick the second option: a big union can send a rep, draft a contract, negotiate with your employer, etc., in exchange for the dues it pays its employees with.