Right, and that lack of bargaining power is because in these days of hefty PCs and standardized software studios can always find other VFX shops, either in the US or abroad, to do a "good enough" job for less.

There's zero chance of requiring studios to only use unionized VFX shops, so unionizing them will simply make them either less competitive (need to charge more to cover guaranteed benefits) or more prone to bankruptcy (cannot meet commitments to workers if there's a shortage of work)