I may not understand something...In such case why is there a vote? If union is a voluntary organization, then why can't people who are for unionizing just open

I may not understand something...In such case why is there a vote? If union is a voluntary organization, then why can't people who are for unionizing just open an organization and join it at will? Who prevents them, it's their own private club, there is no need to vote. People open unions when they choose without asking anybody for permission, why can't these folks?