They aren’t forced to join.
I’m going to be a bit handwavy here:
generally - crew members do not work for a specific company, so the problem of a “Union only” job is perhaps somewhat different. Crew members are all functionally contractors for that one specific production. There are no long term contractual obligations. After that production is over, they have to hunt around for their next gig which reinforces the relationship based job economy of Hollywood labor.
When a production is kicking, off department heads will be hired (for instance, a Director is Photography) and they will hire the crew members below them in the hierarchy. Many department heads will have their regular group of people they call, and it can be difficult to break into the network. Most people who join a union have experience, training, references, etc which help build those connections.
Again, non-union film production happens all the time. The benefits of joining one of the unions are positive enough that most people do, but You don’t have to be in Local 600 to shoot a movie.