The people can't unionize a company that exists for less time than the average unionization campaign takes to form one.
This is why the writers and directors in Hollywood are able to unionize as an industry rather than per-company. (See, in contrast, the recent Starbucks unionizations; each individual location votes and creates their own union with a dozen or so members.)
> Sectoral bargaining was promoted by the National Industrial Recovery Act of 1933, but struck down and replaced by enterprise bargaining under the National Labor Relations Act of 1935. Today industries like screenwriting, hotels, and railroads still see sectoral bargaining predominate.
These temporary movie companies employ writers, actors, directors, electricians etc that belong to unions.