The same isn't true for folks who work directly for Marvel on an ongoing basis.
These temporary movie companies employ writers, actors, directors, electricians etc that belong to unions.
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.)
https://en.wikipedia.org/wiki/Sectoral_collective_bargaining
> 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.
Again, in a special way that is not common in the US, because of the transient nature of movie productions.
https://en.wikipedia.org/wiki/Sectoral_collective_bargaining
Actors and crew have things like the SAG that negotiate with the movie industry as a whole, because it's functionally impossible for them to unionize otherwise. If you're a unionized Starbucks worker, they don't negotiate with the coffee industry, they negotiate with management at your one particular Starbucks store, because that store is an ongoing permanent concern.
VFX artists employed by a permanently-existing organization like Marvel don't need this exception. (Ones employed by a transient production company for a single film would.)
There have been union vfx and animation studios in the past. The industry is broken up into many companies, which is why people talk about unionizing as a whole.
What is the actual point you are trying to make here?