A union is a democratically-run tool. Every tech employee I talk to hates the idea of compensation being based purely on time in role, why would they democratically vote for that to be the rule?
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.