Why do you think that? I'm fairly certain this is incorrect.
AFAIK, North American unions are not generally company specific. They are trade specific in most cases, and in the case of auto-workers it's mostly the UWA.
Depending on the place, its not even always company-wide votes. For instance, there was a push for a few Amazon warehouses to join a union. If those votes had passed, it would have been the workers at that location then represented by the union.
And on top of that, its often only particular jobs that would fall under that union, not all jobs at that facility. That's why the auto companies are doing other layoffs these days, as there are other jobs at the factories that are not a part of the United Auto Workers and are not a part of the strike, but depend on the union workers working for their jobs to not pile up outputs or have no incoming tasks. Its not like all Ford or GM employees are a part of the United Auto Workers.