That's because SAG is a guild, not a union, and it works because every known actor is voluntarily a member, so you can't make a movie without agreeing to their rules.
SAG-AFTRA meets the definition of a labor union under federal and state law. It negotiates working conditions, compensation, and benefits with production companies and studios. It is treated in the manner as other labor unions such as IATSE, IBEW, and Teamsters.