Studios could decide to outsource other talent too. Voice actors, for example. But they choose not to, because they know that if they do, the rest of the actors on set would walk off (and perhaps even on other sets owned by the studio, and perhaps even other unions besides SAG-AFTRA would join in solidarity), and at best they lose a day or two of work and at worst, they lose their movie completely.

Voiceover performance is covered by the Screen Actor's Guild, and I don't hear the same kind of issues again and again with it like I do VFX (even though the work could literally be done on the opposite side of the planet and sent in) I wonder why that is... What would be the main difference there... Hmm

Additionally, there are lots of places outside of Hollywood who are offering very attractive tax cuts, etc., for making movies in their state/province. Pretty easy (and may already be the case, I'm not an expert) for those state-backed contracts torequire union workers.

That's how a lot of government contracting work is done because it is basically win/win/win: the state gets a ton of new revenue from taxes but also from thousands of new workers living and spending in their areas, the workers themselves get well-paying jobs with salaries and benefits they can live on (and hey, maybe even raise a family on), and the consumer gets a new water main, or bridge (or in this case, movie or TV show).